Move Py_buffer_RAII to .h file so SWIG 2 doesnt have to parse it

`struct Py_buffer_RAII` definition uses explicit deleted functions which are not supported by SWIG 2 (only 3).
To get around this I moved this struct to an .h file that is included to avoid being parsed by swig.

Reviewed By: lawrence_danna

Differential Revision: https://reviews.llvm.org/D86381