Drop Python3.6 in CI, setup.cfg, and readme.

[hramezani]

Part of #9437

[bluetech]

Great, thanks @hramezani!

Let's wait a bit to give time for objections in issue #9437, then we can merge.

Only this is that it needs a breaking changelog entry - see instructions here.

[nicoddemus]

Looks good, thanks for working on this!

I checked out your branch, and in rewrite.py:379 we have:

has_flags = sys.version_info >= (3, 7)

has_flags is always True now, so the conditions below which use it can be simplified. 👍

[hramezani]

Thanks @RonnyPfannschmidt, @asottile, and @nicoddemus for the review.
I've addressed all your comments.

[nicoddemus]

Thanks! I missed two paragraphs which should have 3.10 added to them, I will apply them myself.

[bluetech]

I removed py36 from the required checks.

[nicoddemus]

Thanks @hramezani!

[asottile]

[asottile]

I think or makes more sense here (and below)

[hramezani]

I also changed Python 3.6+ and PyPy3 to Python 3.6+ or PyPy3 in Features section of README.
Does it also make sence?

[asottile]

[hoefling]

Out of curiosity, will the silencing of

pytest/pyproject.toml

Lines 31 to 32 in 1131f23

	# produced by python3.6/site.py itself (3.6.7 on Travis, could not trigger it with 3.6.8)."
	"ignore:.*U.*mode is deprecated:DeprecationWarning:(?!(pytest|_pytest))",

still be required when 3.6 is dropped?

[nicoddemus]

Out of curiosity, will the silencing of

Good catch, probably not and can be removed.

[bluetech]

I believe we can merge now, thanks @hramezani!