Move `nogil` to end of function signatures

[cclauss]

Fix build warnings like:

warning: ffpyplayer\pic.pxd:21:61: The keyword 'nogil' should appear at the end of the function signature line. Placing it before 'except' or 'noexcept' will be disallowed in a future version of Cython.

% git grep ' nogil '

https://github.com/cython/cython/blob/e2ef20c527c7d0b1b01d2cbb486c0db92d54d323/Cython/Compiler/Parsing.py#L3148-L3151

Includes the fixes from:

• #191