Consider making contextvars required on Python 3.6

[JayH5]

We're having to do some contortions (see concurrency module or #1257, #1258, #1260) to allow contextvars to not be installed. Rather, let's just make it a requirement on Python 3.6, because:

• It's only required on Python 3.6, which is EoL at the end of 2021. I don't think it's worth putting much effort into reducing one small dependency for this version.
• Lots of things (Trio, various tracing libraries) depend on contextvars already.

[graingert]

it's already there https://github.com/encode/starlette/blob/master/setup.py#L43

[JayH5]

Ah I'm not very smart and mixed up my context<thing>s. I meant contextvars, not contextlib2.