Backport start_tls() support for Python 3.6

[sethmlarson]

This adds support for proxy tunnels for Python 3.6 + asyncio.
Based almost entirely on this comment: urllib3/urllib3#1323 (comment) Massive thanks to @florimondmanca for finding this!

Closes #515.

[lovelydinosaur]

Fantastic stuff!

I guess this PR should also update the test_start_tls_on_socket_stream test case, right?

[yeraydiazdiaz]

LGTM, thanks @sethmlarson! 🌟

[florimondmanca]

Awesome! Approving in advance provided we update the test mentioned by Tom. :) Thanks!

[lovelydinosaur]

At a later date I may look at moving this out to a coverage-excluded compat.py module, so that we can tweak our tests to expect 100% coverage on any single run, without requiring collection across multiple Python versions. (Buit I'm very happy to treat that seperately)

[JayH5]

Removing the type: ignore after changing the if statement to be if sys.version_info >= (3, 7) should pass CI with mypy>=0.730.

[sethmlarson]

Unfortunately we can't assume that loops on Python 3.6 don't have start_tls(). Uvloop supports 3.6 and supports start_tls(). That's interesting that mypy knows about features though! Would an assert work here?

[lovelydinosaur]

getattr would work around any typing issues too.

[JayH5]

Ah I didn't know about uvloop 😞

That's interesting that mypy knows about features though!

It knows about if/else with sys.version_info comparisons but that's all. It's very limited.

Would an assert work here?

I don't think we can assert much about the function at runtime, unfortunately. Could maybe do something with typing.cast but probably not worth the effort.

[florimondmanca]

I removed the version check for 3.6 on the start_tls test, all tests passing!

Mental note that we should refactor sometime in the future to have a compat.py file with this kind of version-specific patching.

Anyway, merging now, thanks @sethmlarson!