Description:

Powerline doesn't function when running inside of a Python virtual environment.

Results in the following error;
Traceback (most recent call last):
File "/usr/bin/powerline", line 22, in <module>
from powerline.lib.encoding import get_preferred_output_encoding
ImportError: No module named powerline.lib.encoding
Traceback (most recent call last):
File "/usr/bin/powerline", line 22, in <module>
from powerline.lib.encoding import get_preferred_output_encoding
ImportError: No module named powerline.lib.encoding

I believe this is because the main `powerline` command uses a `#!/usr/bin/env python` which resolves to the virtualenv python which in turn does not have access to the powerline libraries. This does not affect the other required powerline commands which all use `#!/usr/bin/python` instead.

> head -n1 /usr/bin/powerline-*
==> /usr/bin/powerline-config <==
#!/usr/bin/python

==> /usr/bin/powerline-daemon <==
#!/usr/bin/python

==> /usr/bin/powerline-lint <==
#!/usr/bin/python

==> /usr/bin/powerline-render <==
#!/usr/bin/python

This seems to affect powerline-2.6-2 and above. Prior to this version the main `powerline` script is not a python script with a shebang but instead a compiled executable and so does not suffer interference from the virtualenv.

Affects:
* powerline-2.6-2
* powerline-2.6-3

It might also affect powerline2 but I haven't had a chance to check. I suspect the cause might be something to do with the recent Python 3.7 upgrade in which case it's doubtful this would affect powerline2.

Steps to reproduce:
* Install powerline-2.6-3
* Activate a virtual environment that doesn't have access to system packages and does not have powerline installed.

Workaround:
A quick workaround can be achieved by editing the shebang line for `/usr/bin/powerline` to `#!/usr/bin/python` which will ensure that when ones shell, vim or whatever runs powerline inside a virtualenv it gets the system python (and packages) rather than that of the virtualenv.

Cause:
I'm not entirely sure what caused the change in behaviour.

I believe setuptools is normally responsible for changing the shebangs from `#!/usr/bin/env python` (as they are in the source https://github.com/powerline/powerline/blob/2.6/scripts/powerline-config#L1) to `#!/usr/bin/python`.

Powerline also has some special behaviour in it's `setup.py` which affects the `powerline` script/executable (https://github.com/powerline/powerline/blob/2.6/setup.py). It tries to compile a binary executable version of `powerline` and if that fails falls back to a sh script (if socat, sed and sh can be found by which) or a straight python script otherwise (which seems to be the case since pkg version 2). So based on that I'd say the change in behaviour is due to the failed compilation of the binary, perhaps in turn caused by the upgrade to Python 3.7.

However even if this is the case, I can't understand why the same logic that converts the shebang for the other powerline scripts doesn't come into play for the python version of the powerline script that is included when the compilation fails.

Possible fixes:
* Get the executable version of powerline compiling again.
* Manually tweak the shebang in the PKGBUILD.