bpo-36203: Check callback is callable in PyWeakref_NewRef

[tekknolagi]

The documentation says that PyWeakref_NewRef should raise a
TypeError if the callback is not callable, None, or NULL, but the code
does not reflect the documentation. This PR fixes that.

This should probably be backported to whatever version of Python
changed that documentation.

https://bugs.python.org/issue36203

[the-knights-who-say-ni]

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[tekknolagi]

Huh, weird. I definitely signed it earlier.

[iritkatriel]

The patch needs tests.

[tekknolagi]

Err, what? Why did it close?

[tekknolagi]

Opened #26273 because I cannot reopen this one