Update mitmproxy and dependencies

This updates mitmproxy to 0.10.

netlib is updated due to it having to patch mitmproxy's version. 
lxml and urwid were updated due to newer versions being out.

I added cssutils as it enables beautification of css payloads, but I made it optional.

Regarding issue #25532. I feel it can be closed. Unfortunately the dependencies as currently declared are what mitmproxy's documentation and setup.py indicate are needed.