Merge CI improvements upstream

[pquentin]

I think I prefer to wait until we've caught up with upstream to do this, but I still want to record the list, that I will continue to update:

• Fix TLS 1.3 tests when using ssl from the standard library (a1fa2c2)
• Bump macOS Python versions (important to test 3.7.4 with the above fix)
• Skip one PyPy 2 test (Mysterious PyPy2 test case failure #110)
• Remove newlines after with statements (done as part of Merge from master (June 6th, 2019) #119)
• Python 2 coverage uploads
• setup.cfg

Things that we don't get right either:

• Is the no brotli test running somewhere?
• Can we drop the warnings configuration when launching pytest with the latest pytest warning changes? This would allow to use --disable-warnings
• Fix lines merged by black (using rg '" "')
• Use cls in setup_class
• Remove useless proxy_from_url redirection (it's part of the public API)

Things that are specific to us:

• We no longer use pytest-random-order: either configure it or drop it
• Remove enforce_content_length=False
• AppVeyor does not upload its coverage results to codecov

[RatanShreshtha]

I think we should keep pytest-random-order and configure it in our CI, I will be happy to open a PR for that.

[pquentin]

Thanks, that would be useful indeed 👍