bpo-35081: Move Py_BUILD_CORE code to internal/mem.h

[vstinner]

• Add #include "internal/mem.h" to C files using
  _PyMem_SetDefaultAllocator().
• Include/internal/mem.h now requires Py_BUILD_CORE to be defined.

https://bugs.python.org/issue35081

[bedevere-bot]

⚠️⚠️⚠️ Buildbot failure ⚠️⚠️⚠️

Hi! The buildbot AMD64 FreeBSD 10-STABLE Non-Debug 3.x has failed when building commit 2be00d9.

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/130) and take a look at the build logs.
4. Check if the failure is related to this commit (2be00d9) 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/130

Click to see traceback logs

Traceback (most recent call last):
  File "/usr/home/buildbot/python/3.x.koobs-freebsd10.nondebug/build/Lib/test/test_time.py", line 515, in test_process_time
    self.assertGreaterEqual(stop - start, min_time)
AssertionError: 0.005377999999999883 not greater than or equal to 0.015

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

Ran 56 tests in 3.407s

FAILED (failures=1, skipped=2)