refactor: replace none equality checks with is/is not equivalents

[cpcloud]

This PR implements proper None checks using is/is not, contains code regenerated from https://github.com/NVIDIA/cuda-python-private/pull/146.

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[leofang]

/ok to test

[mdboom]

Just out of curiosity, does this change the code Cython generates, or is it already smart enough to know that a None comparison can be done with a simple pointer comparison?

[cpcloud]

It does change the code, because == (and !=) must produce behavior equivalent to PyObject_RichCompare. Cython generates the rich comparison call for == and != and generates C code like this:

__pyx_v_x != Py_None

when is not is used to compare None.