gltbx: Windows failure with Python => 3.8

[bkpoon]

On Windows, there is an issue with loading the gltbx extensions in Python 3.8 and greater. Python 3.7 is not affected.

This is probably due to the change in DLL resolution starting with Python 3.8 (https://github.com/python/cpython/issues/80266). The easiest solution is to migrate to pyopengl instead of debugging DLL resolution issues. One consequence is that OpenGL functions from pyopengl do not have keyworded arguments, so the keywords will have to be removed (but keep the arguments).

@dagewa Is it possible to add pyopengl as a dependency to CCP4?