bpo-36724: Add _PyWarnings_Fini()

[vstinner]

Py_FinalizeEx() now clears _PyRuntime.warnings variables and
_PyRuntime.exitfuncs.

Changes:

• Add _PyWarnings_Fini(): called by Py_FinalizeEx()
• call_ll_exitfuncs() now clears _PyRuntime.exitfuncs while iterating
  on it (on backward order).

https://bugs.python.org/issue36724

[bedevere-bot]

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

[bedevere-bot]

⚠️⚠️⚠️ Buildbot failure ⚠️⚠️⚠️

Hi! The buildbot AMD64 FreeBSD 10-STABLE Non-Debug 3.x has failed when building commit 87d23a0.

What do you need to do:

1. Don't panic.
2. Check the buildbot page in the devguide if you don't know what the buildbots are or how they work.
3. Go to the page of the buildbot that failed (https://buildbot.python.org/all/#builders/167/builds/844) and take a look at the build logs.
4. Check if the failure is related to this commit (87d23a0) or if it is a false positive.
5. If the failure is related to this commit, please, reflect that on the issue and make a new Pull Request with a fix.

You can take a look at the buildbot page here:

https://buildbot.python.org/all/#builders/167/builds/844

Click to see traceback logs

Traceback (most recent call last):
  File "/usr/home/buildbot/python/3.x.koobs-freebsd10.nondebug/build/Lib/test/_test_multiprocessing.py", line 1873, in test_default_timeout
    self.assertEqual(len(results), barrier.parties)
AssertionError: 0 != 5

----------------------------------------------------------------------

Ran 345 tests in 281.373s

FAILED (failures=1, skipped=29)