Cython support

[MichaReiser]

Originally requested in #1079

Requires a custom parse mode and AST extensions to support Cython specific syntax. See https://cython.readthedocs.io/en/latest/src/userguide/language_basics.html

[MarcoGorelli]

They provide StringParseContext, which gives something like an AST but with lots of missing parts (I had to do quite some working around it in cython-lint...sharing in case it's useful as a starting point: https://github.com/MarcoGorelli/cython-lint/blob/9e1d65daed6a23ff32e1893b749607432fd6d8f1/cython_lint/cython_lint.py#L374-L689)

[carmocca]

isort 5 introduced support for Cython: https://pycqa.github.io/isort/docs/major_releases/introducing_isort_5.html. This could be an easier first step towards full support.

[KelSolaar]

I would also like to see the formatter working with Cython.

(Pretty Please)