Expose contextvars CPython implementation

[pfreixes]

Hi,

I'm planning on use contextvars [1] within a Cython code and I would like to have access to the CPython implementation instead of accessing through the generic Python module.

Does it make sense?

In such case, I could try to work on a PR that would define the methods as a new include file behind the generic Include/cpython directory.

[1] https://docs.python.org/dev/library/contextvars.html#module-contextvars

[scoder]

Yes, I think covering the C-API in a .pxd file would be a good first step. Eventually, Cython should learn how to use context variables internally and make them available as efficient context managers.

[scoder]

I added convenience getter functions in ff16389.