Consider dropping Python 3.6 support

[michaeloliverx]

Python 3.6 is now considered legacy¹, we should remove support and drop any compatibility shims in place:

httpcore/httpcore/_compat.py

Lines 1 to 6 in ebf0684

	# `contextlib.asynccontextmanager` exists from Python 3.7 onwards.
	# For 3.6 we require the `async_generator` package for a backported version.
	try:
	from contextlib import asynccontextmanager # type: ignore
	except ImportError:
	from async_generator import asynccontextmanager # type: ignore # noqa

Footnotes

1. The release you are looking at is Python 3.6.0, the initial feature release for the legacy 3.6 series which has now reached end-of-life and is no longer supported. ↩

[tomchristie]

Yup to this, given encode/httpx#2097.