bpo-23403: Bump pickle.DEFAULT_PROTOCOL to 4

[ambv]

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

[serhiy-storchaka]

Please update docstrings, the module documentation, and What's New.

[serhiy-storchaka]

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

[ambv]

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?

[serhiy-storchaka]

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

Regenerate Argument Clinic files and fix tests.

[ambv]

@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.

[serhiy-storchaka]

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

[serhiy-storchaka]

And maybe add a versionchanged directive somewhere in pickle.rst?

[serhiy-storchaka]

Tiny nit: it is preferable to use an en-dash for the version range.

Python 3.0--3.7

[ambv]

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

[bedevere-bot]

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