Better support for typing_extensions module

[Zac-HD]

The typing_extensions module "contains both backports of new types added to the typing module, as well as experimental types that will eventually be added to the typing module".

What makes this relevant to us is that we have a bunch of internal code that looks for these types specifically in the typing module, e.g.:

hypothesis/hypothesis-python/src/hypothesis/strategies/_internal/types.py

Lines 84 to 88 in cd9d7b3

	def is_typing_literal(thing):
	return (
	hasattr(typing, "Literal")
	and getattr(thing, "__origin__", None) == typing.Literal
	)

So if you're using typing.Literal in Python 3.8, that works. But if you're using typing_extensions.Literal in Python 3.7 - or 3.8 for compatibility - you're out of luck.

Going through and inserting checks to handle typing_extensions would improve our inference support for projects like LibCST - I found this while working on an extension to Zac-HD/hypothesmith#2, but obviously it's a generally useful feature.

The main cost will be setting up tests for another optional integration to check that our non-typing_extensions-specific features work whether it's installed or not, that they work when installed, and ideally that we don't import it if it's not already in use.