While thinking about adding a test I realized we actually no longer need a custom __sizeof__() implementation now as the value of ob_size should be accurate. So, I've removed that, and now the tests in test_sys.py should be sufficient to exercise the change. For good measure I've added an extra case to cover two differently sized generators.

Honestly, though this has left me even more puzzled as to whether this was intentional or not. It should be noted that this change will increase the size of all generator objects (e.g. by 8 bytes on x86-64 Linux systems). The addition of the custom __sizeof__() makes it looks like we were trying to avoid exposing the lack of correct ob_size, maybe to save the need for the storage? However, that only works for Python code, not users of the C-API.

No idea. @markshannon was who wrote it, I'll wait for his input.

See #126596 (comment)