[DISC] Use type annotations in cython

[jbrockmendel]

Regardless of whether run in py2/py3, cython supports modern type annotations. A lot of _libs could be made valid python by using these.

Upsides:

• linting!
• get experience with these, will help in deciding if we want to use them in the non-cython code-base.
• may lower barrier-to-entry for contributors not comfortable with cython

Downsides:

• If it works, don't fix it
• Until fully implemented, we'd have mix-and-match conventions
• cython's pure-python mode has more rough edges than its regular mode
• in some cases the valid-python versions get pretty verbose:

cdef inline bint is_integer_object(object obj) nogil:
    cdef:
         int foo

vs.

@cython.cfunc
@cython.inline
@cython.nogil
def is_integer_object(tz: object) -> bint:
    foo: int

The main upside I see is the linting. My inclination is to try to implement it in a subset of files where we can get close to pure-python as a test run.

Thoughts?

[WillAyd]

Is this a new feature? If it's as advertised I'm +1 as it would probably even make it easier for newcomers to understand the Cython code base

[gfyoung]

+1 from me as well.

[jbrockmendel]

Opened cython#2529 to discuss some syntax combinations that are parsed incorrectly (or not supported, but not made explicit in the docs). I think this is worth revisiting a couple of cython releases down the road, but not just yet. Closing.