This commit replaces our WSGI implementation with a new ASGI one,
which then uses `asgiref`'s compatibility mode to still support WSGI applications.
The ASGI implementation is a bit bare-bone, but good enough for our purposes.
The major changes are:
- We now support ASGI apps.
- Instead of taking connections out of mitmproxy's normal processing,
we now just set flow.response and let things continue as usual.
This allows users to see responses in mitmproxy, use the response hook
to modify app responses, etc. Also important for us,
this makes the new implementation work for shenanigans like sans-io.
- restructure examples (fix#4031)
- remove example dependencies from setup.py,
we do not need special dependencies for our supported addons.
- unify how we generate docs from code
- improve example docs
