gh-83915: Deprecate the readinto() fallback path in _pickle.c

[ZackerySpytz]

https://bugs.python.org/issue39734

• Issue: Deprecate readinto() fallback path in _pickle.c #83915

[ZackerySpytz]

I think the warning message could be better.
This PR still needs a news entry (if it is accepted).

[pitrou]

Thank you for doing this. It looks good on the principle.

[bedevere-bot]

A Python core developer has requested some changes be made to your pull request before we can consider merging it. If you could please address their requests along with any other requests in other reviews from core developers that would be appreciated.

Once you have made the requested changes, please leave a comment on this pull request containing the phrase I have made the requested changes; please review again. I will then notify any core developers who have left a review that you're ready for them to take another look at this pull request.

[iritkatriel]

I have made the requested changes; please review again.

Requested changes were made

[iritkatriel]

With the change the test fails for the Python version unpickler:

======================================================================
FAIL: test_multiple_unpicklings_minimal (test.test_pickle.PyPicklerUnpicklerObjectTests.test_multiple_unpicklings_minimal)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/Users/iritkatriel/src/cpython/Lib/test/pickletester.py", line 3710, in test_multiple_unpicklings_minimal
    with self.assertWarns(DeprecationWarning):
AssertionError: DeprecationWarning not triggered

[bedevere-bot]

A Python core developer has requested some changes be made to your pull request before we can consider merging it. If you could please address their requests along with any other requests in other reviews from core developers that would be appreciated.

Once you have made the requested changes, please leave a comment on this pull request containing the phrase I have made the requested changes; please review again. I will then notify any core developers who have left a review that you're ready for them to take another look at this pull request.

[python-cla-bot]

All commit authors signed the Contributor License Agreement.

[github-actions]

This PR is stale because it has been open for 30 days with no activity.