Allow not None in cdef and cpdef function arguments

[jakirkham]

It would be useful to allow not None on cdef and cpdef function arguments as it works with def function arguments.

ref: https://cython.readthedocs.io/en/latest/src/userguide/extension_types.html#extension-types-and-none

[McSinyx]

Coming from #3328: For future design, I suggest using Python functions, use type annotation, e.g. def func(b: bytes). Explicitly allowing None can be done using def func(b: typing.Optional[bytes]) in Python, and Cython function declaration may want to accept this syntax.