Merge CI improvements upstream

[pquentin]

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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 (https://github.com/python-trio/urllib3/pull/107/commits/a1fa2c2be9657dbe602595f8c7e488df8b431a8d)~
• ~Bump macOS Python versions (important to test 3.7.4 with the above fix)~
• ~Skip one PyPy 2 test (#110)~
• ~Remove newlines after with statements (done as part of #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 :+1: