Description:
As of Lutris 5.8.3-1 Lutris will fail to start with and error `ModuleNotFoundError: No module named 'lxml'`

This was reported in the Lutris Discord and one of the Lutris team answered that the lxml package needs to be installed. The OP of the problem stated that the issue was fixed with the installation of the package.

I tested this myself and got the error above. After installing `python-lxml` Lutris started with no errors.

It would seem the Lutris team added some function that requires `python-lxml` but as of now it is not included as a dependency.

Additional info:
* package version(s) 0.5.8.3-1
* If it is relevant I am using a Nvidia card, in case it doesn't affect AMD, however the output would not suggest that it is hardware dependent.
* I am using KDE in case the GTK-Warnings are related.

Steps to reproduce:

Start Lutris from the terminal, it should fail to start with the final error being ModuleNotFoundError: No module named 'lxml'

Here is my entire terminal output for verfication.

```(lutris:4955): Gtk-WARNING **: 21:04:21.766: Theme parsing error: gtk.css:68:35: The style property GtkButton:child-displacement-x is deprecated and shouldn't be used anymore. It will be removed in a future version

(lutris:4955): Gtk-WARNING **: 21:04:21.766: Theme parsing error: gtk.css:69:35: The style property GtkButton:child-displacement-y is deprecated and shouldn't be used anymore. It will be removed in a future version

(lutris:4955): Gtk-WARNING **: 21:04:21.766: Theme parsing error: gtk.css:73:46: The style property GtkScrolledWindow:scrollbars-within-bevel is deprecated and shouldn't be used anymore. It will be removed in a future version
Traceback (most recent call last):
File "/usr/bin/lutris", line 52, in <module>
from lutris.gui.application import Application # pylint: disable=no-name-in-module
File "/usr/lib/python3.9/site-packages/lutris/gui/application.py", line 41, in <module>
from lutris.gui.installerwindow import InstallerWindow
File "/usr/lib/python3.9/site-packages/lutris/gui/installerwindow.py", line 17, in <module>
from lutris.installer import interpreter
File "/usr/lib/python3.9/site-packages/lutris/installer/interpreter.py", line 14, in <module>
from lutris.installer.installer import LutrisInstaller
File "/usr/lib/python3.9/site-packages/lutris/installer/installer.py", line 16, in <module>
from lutris.services import get_services
File "/usr/lib/python3.9/site-packages/lutris/services/__init__.py", line 2, in <module>
from lutris.services.gog import GOGService
File "/usr/lib/python3.9/site-packages/lutris/services/gog.py", line 9, in <module>
from lxml import etree
ModuleNotFoundError: No module named 'lxml'
[zookeeper@RGBz ~]$ lutris

(lutris:5676): Gtk-WARNING **: 21:09:11.569: Theme parsing error: gtk.css:68:35: The style property GtkButton:child-displacement-x is deprecated and shouldn't be used anymore. It will be removed in a future version

(lutris:5676): Gtk-WARNING **: 21:09:11.569: Theme parsing error: gtk.css:69:35: The style property GtkButton:child-displacement-y is deprecated and shouldn't be used anymore. It will be removed in a future version

(lutris:5676): Gtk-WARNING **: 21:09:11.569: Theme parsing error: gtk.css:73:46: The style property GtkScrolledWindow:scrollbars-within-bevel is deprecated and shouldn't be used anymore. It will be removed in a future version
2021-01-24 21:09:11,794: Your version of python-magic is too old.
2021-01-24 21:09:11,881: Initializing lutris
2021-01-24 21:09:13,812: Downloading DXVK releases to /home/zookeeper/.local/share/lutris/runtime/dxvk/dxvk_versions.json
2021-01-24 21:09:13,813: DXVK v1.7.3L-03f11ba not available, downloading...
2021-01-24 21:09:57,466: Writing to /home/zookeeper/.local/share/lutris/runtime/dxvk/dxvk-1.7.3L-03f11ba.tar.gz
2021-01-24 21:09:57,609: Runtime updated. Initialization complete.
2021-01-24 21:09:57,610: Lutris 0.5.8.3
2021-01-24 21:09:57,610: Using NVIDIA drivers 460.32.03 for x86_64
2021-01-24 21:09:57,610: GPU: GeForce RTX 2070 SUPER
2021-01-24 21:09:57,610: GPU: 10DE:1E84 1458:4001 (nvidia drivers)

(lutris:5676): Gtk-WARNING **: 21:09:57.658: Theme parsing error: gtk.css:68:35: The style property GtkButton:child-displacement-x is deprecated and shouldn't be used anymore. It will be removed in a future version

(lutris:5676): Gtk-WARNING **: 21:09:57.658: Theme parsing error: gtk.css:69:35: The style property GtkButton:child-displacement-y is deprecated and shouldn't be used anymore. It will be removed in a future version

(lutris:5676): Gtk-WARNING **: 21:09:57.658: Theme parsing error: gtk.css:70:34: The style property GtkCheckButton:indicator-size is deprecated and shouldn't be used anymore. It will be removed in a future version

(lutris:5676): Gtk-WARNING **: 21:09:57.658: Theme parsing error: gtk.css:71:36: The style property GtkCheckMenuItem:indicator-size is deprecated and shouldn't be used anymore. It will be removed in a future version

(lutris:5676): Gtk-WARNING **: 21:09:57.658: Theme parsing error: gtk.css:73:46: The style property GtkScrolledWindow:scrollbars-within-bevel is deprecated and shouldn't be used anymore. It will be removed in a future version

(lutris:5676): Gtk-WARNING **: 21:09:57.658: Theme parsing error: gtk.css:76:30: The style property GtkExpander:expander-size is deprecated and shouldn't be used anymore. It will be removed in a future version
2021-01-24 21:10:01,419: Shutting down Lutris
[zookeeper@RGBz ~]$ lutris

(lutris:5813): Gtk-WARNING **: 21:10:18.613: Theme parsing error: gtk.css:68:35: The style property GtkButton:child-displacement-x is deprecated and shouldn't be used anymore. It will be removed in a future version

(lutris:5813): Gtk-WARNING **: 21:10:18.613: Theme parsing error: gtk.css:69:35: The style property GtkButton:child-displacement-y is deprecated and shouldn't be used anymore. It will be removed in a future version

(lutris:5813): Gtk-WARNING **: 21:10:18.613: Theme parsing error: gtk.css:73:46: The style property GtkScrolledWindow:scrollbars-within-bevel is deprecated and shouldn't be used anymore. It will be removed in a future version
Traceback (most recent call last):
File "/usr/bin/lutris", line 52, in <module>
from lutris.gui.application import Application # pylint: disable=no-name-in-module
File "/usr/lib/python3.9/site-packages/lutris/gui/application.py", line 41, in <module>
from lutris.gui.installerwindow import InstallerWindow
File "/usr/lib/python3.9/site-packages/lutris/gui/installerwindow.py", line 17, in <module>
from lutris.installer import interpreter
File "/usr/lib/python3.9/site-packages/lutris/installer/interpreter.py", line 14, in <module>
from lutris.installer.installer import LutrisInstaller
File "/usr/lib/python3.9/site-packages/lutris/installer/installer.py", line 16, in <module>
from lutris.services import get_services
File "/usr/lib/python3.9/site-packages/lutris/services/__init__.py", line 2, in <module>
from lutris.services.gog import GOGService
File "/usr/lib/python3.9/site-packages/lutris/services/gog.py", line 9, in <module>
from lxml import etree
ModuleNotFoundError: No module named 'lxml'
```