Do not access PyFrameObject fields directly on Python 3.9+

[musicinmybrain]

Fixes a Python 3.11 incompatibility. Still accesses PyCodeObject fields directly.

Description of Change(s)

Fixes Issue(s)

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[musicinmybrain]

Adjusted based on @hroncok’s good suggestion. Also adjusted the C-style cast style to match others in the codebase ((Foo *) var → (Foo *)var).

[musicinmybrain]

Now the patch defines a static inline compatibility version of PyFrame_GetCode on older Pythons that don’t have it, as suggested in https://docs.python.org/3.11/whatsnew/3.11.html#id6. I think the result is a bit cleaner.

Now PyFrame_GetCode is used exactly where it is available: Python 3.9.0b1 and later, again based on example in the CPython documentation.

I’ve made the change in a separate commit for clarity, but it can of course be squashed.

[sunyab]

Filed as internal issue #USD-7464