gh-135755: Use private names (_Py*) for header file guards new in 3.14

[encukou]

These are private; let's name new ones accordingly.

• Issue: Undocumented 3.14 C API #135755

[miss-islington-app]

Thanks @encukou for the PR 🌮🎉.. I'm working now to backport this PR to: 3.14.
🐍🍒⛏🤖

[bedevere-app]

GH-135976 is a backport of this pull request to the 3.14 branch.

[bedevere-bot]

⚠️⚠️⚠️ Buildbot failure ⚠️⚠️⚠️

Hi! The buildbot AMD64 Ubuntu Shared 3.14 (tier-1) has failed when building commit 665a682.

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/#/builders/1802/builds/251) and take a look at the build logs.
4. Check if the failure is related to this commit (665a682) 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/#/builders/1802/builds/251

Failed tests:

• test_external_inspection

Failed subtests:

• test_only_active_thread - test.test_external_inspection.TestGetStackTrace.test_only_active_thread

Summary of the results of the build (if available):

==

Click to see traceback logs

Traceback (most recent call last):
  File "/srv/buildbot/buildarea/3.14.bolen-ubuntu/build/Lib/test/test_external_inspection.py", line 991, in test_only_active_thread
    self.assertEqual(len(gil_traces), 1, "Should have exactly one GIL holder")
    ~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
AssertionError: 0 != 1 : Should have exactly one GIL holder