test.test_zoneinfo.test_zoneinfo.ExtensionBuiltTest.test_cache_location fails if `_zoneinfo` Cextension isn't built

Bug report

Bug description:

I was doing some testing to check that Python without optional C extensions remains functional, and I've noticed that the following test fails if _zoneinfo is not built:

======================================================================
FAIL: test_cache_location (test.test_zoneinfo.test_zoneinfo.ExtensionBuiltTest.test_cache_location)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/mgorny/git/cpython/Lib/test/test_zoneinfo/test_zoneinfo.py", line 1931, in test_cache_location
    self.assertFalse(hasattr(c_zoneinfo.ZoneInfo, "_weak_cache"))
    ~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
AssertionError: True is not false

FWICS the zoneinfo module tests unconditionally assume that zoneinfoimported with no "blocked" modules. While most of the time, this shouldn't matter, I suppose it would be cleaner to haveget_modules()returnNoneasc_moduleif_zoneinfo` is not importable.

I can try making a PR for this if you agree with this approach (and skipping the duplicate-then C extension tests).

CC @pganssle

CPython versions tested on:

3.13, CPython main branch

Operating systems tested on:

Linux

Linked PRs

• gh-131104

Start in Lib/test/test_zoneinfo/test_zoneinfo.py at ExtensionBuiltTest.test_cache_location and inspect get_modules(), especially how it handles an unavailable _zoneinfo module. Verify the zoneinfo tests when the optional C extension is not built, including whether duplicate C-extension tests are skipped. Done means the test suite no longer assumes _zoneinfo is importable and the reported failure is resolved.