Drop support for and remove references to EOL Python 3.6

[H-Shay]

As the title states, removes references to python 3.6 and upgrades to Python 3.7 where necessary.

[H-Shay]

Requires matrix-org/sytest#1181, which is blocked by matrix-org/sytest#1114, but there is a PR open to fix that: matrix-org/sytest#1182

[H-Shay]

Looks like Pillow 4 is not compatible with Python 3.7: https://pillow.readthedocs.io/en/latest/installation.html
Can we bump the version? Looks like we will need to bump it to at least 5.2

[DMRobertson]

Thanks for seeing this through. I've tried to elaborate on why we needed to bump things for olddeps. Hope it's useful, as a historical record if nothing else.

Two small things:

• Have we run the lxml version bump past package maintainers?
• The twisted_trunk workflow speaks of Python 3.6. Could probably change this to "3.7" or "3.x"?

[H-Shay]

I've shouted in the packaging room, haven't heard of issues, so hopefully this is good to go.

[DMRobertson]

LGTM. Thanks for your diligence on this one!

[anoadragon453]

Heads up that there are a couple checks for Python 3.7+ that can be removed here:

synapse/synapse/app/_base.py

Lines 467 to 480 in 20c6d85

	# We now freeze all allocated objects in the hopes that (almost)
	# everything currently allocated are things that will be used for the
	# rest of time. Doing so means less work each GC (hopefully).
	#
	# This only works on Python 3.7
	if platform.python_implementation() == "CPython" and sys.version_info >= (3, 7):
	gc.collect()
	gc.freeze()
	# Speed up shutdowns by freezing all allocated objects. This moves everything
	# into the permanent generation and excludes them from the final GC.
	# Unfortunately only works on Python 3.7
	if platform.python_implementation() == "CPython" and sys.version_info >= (3, 7):
	atexit.register(gc.freeze)

[clokep]

I think we need to still check the platform implementation is CPython here -- I think pypy doesn't have these APIs.

[callahad]

pypy has gc.collect but not gc.freeze

In the interest of duck typing, we should probably check hasattr(gc, 'freeze') instead of examining python_implementation()

(Or we could say we're standardizing on CPython and rip the Pypy bits out of our codebase; no shame in that)

[clokep]

Either of those seems ok, just a shame to delete something that's working.

[H-Shay]

Great, I will re-implement the relevant parts of this check. Since this is already merged, what would be the best way of going about that? Opening a new PR restoring that functionality?

[clokep]

Great, I will re-implement the relevant parts of this check. Since this is already merged, what would be the best way of going about that? Opening a new PR restoring that functionality?

Exactly! It probably makes sense to just use the same changelog entry so they'll get merged by towncrier.