The current mitmproxy release doesn't work because it depends on the python2-h2 2.1.x series, while python2-h2 is currently at version 2.2.1.
The mitmproxy dependency is unversioned, so there's no sign of the incompatibility.

[bardo@corwin ~]$ mitmproxy
Traceback (most recent call last):
File "/usr/bin/mitmproxy", line 5, in <module>
from pkg_resources import load_entry_point
File "/usr/lib/python2.7/site-packages/pkg_resources/__init__.py", line 2905, in <module>
@_call_aside
File "/usr/lib/python2.7/site-packages/pkg_resources/__init__.py", line 2891, in _call_aside
f(*args, **kwargs)
File "/usr/lib/python2.7/site-packages/pkg_resources/__init__.py", line 2918, in _initialize_master_working_set
working_set = WorkingSet._build_master()
File "/usr/lib/python2.7/site-packages/pkg_resources/__init__.py", line 644, in _build_master
return cls._build_from_requirements(__requires__)
File "/usr/lib/python2.7/site-packages/pkg_resources/__init__.py", line 657, in _build_from_requirements
dists = ws.resolve(reqs, Environment())
File "/usr/lib/python2.7/site-packages/pkg_resources/__init__.py", line 834, in resolve
raise DistributionNotFound(req, requirers)
pkg_resources.DistributionNotFound: The 'h2<2.2,>=2.1.0' distribution was not found and is required by mitmproxy
[bardo@corwin ~]$ pacman -Qi mitmproxy | grep -e Version -e Depends
Version : 0.16-2
Depends On : python2-netlib python2-h2 python2-tornado python2-configargparse python2-pyperclip python2-blinker python2-pyparsing python2-html2text python2-construct python2-six python2-pillow python2-watchdog python2-click python2-urwid python2-lxml python2-setuptools
[bardo@corwin ~]$ pacman -Q python2-h2
python2-h2 2.2.1-1