[hold until 3.6 EOL] Bump min Python version from 3.6.2 to 3.7.2

[c0llab0rat0r]

1. Drop support for 3.5 and 3.6
2. Remove conditional shimming for 3.5 and 3.6
3. Change some typing from comment style to 3.7 style
4. Suppress the mypy warnings in returns_single_item and returns_multiple_items that were causing a pre-existing build failure
5. Run some of the build steps on focal (defaults to Python 3.8) that were running on bionic (defaults to Python 3.6)

There's still quite a few typing warning suppression; this commit doesn't attempt to include a sweeping correction to those. Rather, it gets the library up to Python 3.7, which will make a sweeping correction easier.

[c0llab0rat0r]

@ntninja I'm pretty close on this, but as you can see, I'm having some trouble with the last mile.

I ran into two problems:

1. pypy3 build did not work anymore. I believe this to be caused by travis only having Python 3.6, and not having a newer version of pypy that supports Python 3.7 syntax. I resolved this by commenting pypy out of the build
2. httpcore does not import when running the httpx build. I tried to see if I could get the http_requests flavor to work instead (there's a code change to the imports to sidestep httpx) and that resulted in coverage errors (89% < 90%)

I feel like I'm close but I need some help to push it across the finish line.

[c0llab0rat0r]

Holy hell, the build finally "passes" - except for the style linting, which is fixed in #257, and should be merged first.

A code review would be much appreciated.

[c0llab0rat0r]

I would expect this change to get squashed to remove some of my commit noise and silly comments.

[ntninja]

Thank you, this looks pretty good overall! As mentioned, Python 3.6 support is still important through so that will have to be added back. Otherwise it's only minor stuff.

[c0llab0rat0r]

Build is green after rebase and incorporating code review feedback.

It doesn't have the 3.7 -> 3.6 code review feedback yet, which I'll get to once I have a larger block of time available.

I'll incorporate smaller code review feedback items as they're posted.

[ntninja]

Build is green after rebase and incorporating code review feedback.

Awesome! Thank you! Don't overwork yourself or anything (just let me know if you don't feel like finishing something)! 😉

[c0llab0rat0r]

Transient docker failures on the Windows builds.

[c0llab0rat0r]

Another transient docker failure on a Windows build.

[c0llab0rat0r]

Another transient Windows build failure, this time with a link:

https://travis-ci.com/github/ipfs-shipyard/py-ipfs-http-client/jobs/497459752

They all looked like this one:

 	# Wait for daemon process to disappear

	for _ in range(10000):
		if not daemon_is_running():
			break

		time.sleep(0.001)

	# Daemon should not be running anymore
>   	assert not daemon_is_running()
E    assert not True
E     +  where True = <function test_daemon_stop.<locals>.daemon_is_running at 0x0000020AFD70BDC8>()
test\functional\test_miscellaneous.py:47: AssertionError

============================== warnings summary ===============================

test\functional\test_miscellaneous.py:23

C:\Users\travis\build\ipfs-shipyard\py-ipfs-http-client\test\functional\test_miscellaneous.py:23: PytestUnknownMarkWarning: Unknown pytest.mark.last - is this a typo?  You can register custom marks to avoid this warning - for details, see https://docs.pytest.org/en/latest/mark.html

@pytest.mark.last