gh-93627: Align Python implementation of pickle with C implementation of pickle

[eendebakpt]

In this PR we align the python and C implementation of pickle module. In this PR we have chosen to modify all implementations to follow the c implementation of pickle conventions: if a method like __reduce_ex_ or __reduce__ is set to None, a TypeError is raised.

Since the behaviour of the python and c implemenations is not equal, any PR aliging the two cannot be fully backwards compatible. The impact seems limited:

• No changes are required to the tests (although a few tests are added to verify the new behaviour)
• The changes impact cases that seem rare (e.g. setting the __reduce_ex__ attribute of a class to None, instead of creating a method)

Notes:

• Changes to align the copy module have been removed from the PR.

• Issue: Inconsistency in handling None handlers in the Python and C implementations of pickle #93627