gh-145856: Fix plistlib.dumps() skipkeys behavior with mixed key types

[ankhikarmakar]

When both skipkeys=True and sort_keys=True were passed to plistlib.dump() / plistlib.dumps(), the sort ran before non-string keys were filtered out, so a dictionary with mixed key types raised TypeError instead of dropping the non-string keys per the documented contract.

This PR filters non-string keys before sorting in all three write_dict sites — the XML writer (PlistWriter.write_dict) and both passes of the binary writer (_BinaryPlistWriter._flatten and _BinaryPlistWriter._write).

Test plan:

• Extended test_skipkeys to cover sort_keys=True in addition to sort_keys=False.
• Added test_skipkeys_with_sort_keys_mixed_types (the reproducer from the issue) — fails without the fix, passes with it, across all formats.
• Existing test_plistlib (72 tests) still passes.

• Issue: plistlib.dumps() raises TypeError when skipkeys=True and dict has mixed key types #145856

[python-cla-bot]

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[encukou]

This looks reasonable & complete, and it even simplifies the implementation a bit.
Thank you!

[encukou]

Now it fails. Could you fix the test_keys_no_string test?

Also, please do not force-push to CPython PRs, it makes the PR hard to review.

[ankhikarmakar]

Reverted to the old commit. Please review @encukou .

[serhiy-storchaka]

There was also equivalent PR #150109, opened a week earlier, which was closed to give the author of the issue chance to submit his own PR, which he didn't.

[serhiy-storchaka]

I suggest to add @VanshAgarwal24036 as a co-author. He created an equivalent PR earlier.

[serhiy-storchaka]

Looking at this again I see several issues.

• We create a copy of items even if sort_keys is false. If the dict was huge, this can consume significant amount of memory. Perhaps we should make a copy only if it is needed for sorting.
• We can get different error than "keys must be strings" if sort_keys is true, skipkeys is false, and the dict contains mixed keys. We should check types before sorting if skipkeys is false.

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