This makes performance better and produces shorter pickles. This change is backwards compatible up to the oldest currently supported version of Python (3.4).

https://bugs.python.org/issue23403

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Since this is a reStructuredText, we can add links: :mod:`pickle`, :pep:`3154`.

Lib/test/pickletester.py:AbstractPicklerUnpicklerObjectTests.test_clear_pickler_memo is failing for CPicklerUnpicklerObjectTests only. This seems like a bug to me since it passes on the same protocol in the pure-Python implementation (PyPicklerUnpicklerObjectTests).

@pitrou Could you take a look?

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There is other mention of the default protocol 3 in the module documentation.

Regenerate Argument Clinic files and fix tests.

@serhiy-storchaka, updated the remaining comment on default protocol 3, regenerated Argument Clinic. For the failing test, see my comment above, I'm not sure what the fix should be here.

I think there is a bug in C implementation. Opened bpo-33209.

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Responded to remaining nits and rebased to include GH-6363. All tests pass now.

@ambv: Please replace # with GH- in the commit message next time. Thanks!