gh-114685: PyBuffer_FillInfo now raises on PyBUF_{READ,WRITE}

[sobolevn]

• Issue: _imp.get_frozen_object possible incorrect use of PyBUF_READ #114685

[serhiy-storchaka]

See #60198. PyBuffer_FillInfo() with NULL as exporter is a fragile thing.

[serhiy-storchaka]

Thank you.

Yet one nitpick. The code relies on the fact that b"abc" still exists after returning from buffer_fill_info(). It is so in CPython, but this is an implementation detail. It would be more reliable to explicitly keep a reference to it:

source = b'abc'
...
buffer_fill_info(source, ...)

[sobolevn]

Thank you! I always learn a lot from your reviews, very helpful.

[serhiy-storchaka]

LGTM.