gh-158684: Remove ineffective lazy import of sysconfig from ctypes

[sillyfellow21]

Fixes gh-158684.

Root cause

Lib/ctypes/__init__.py declared a lazy import of sysconfig:

lazy import sysconfig as _sysconfig

but the only uses of _sysconfig were at module scope, inside the android and cygwin branches of the pythonapi setup:

elif _sys.platform == "android":
    pythonapi = PyDLL(_sysconfig.get_config_var("LDLIBRARY"))
elif _sys.platform == "cygwin":
    pythonapi = PyDLL(_sysconfig.get_config_var("DLLLIBRARY"))

Reading a lazily imported name reifies that import immediately, so the lazy import was not lazy at all on those two platforms, and on every other platform nothing ever resolved it: it just sat in sys.lazy_modules forever. Either way the laziness bought nothing, and retaining an unresolvable pending entry is the bug reported in gh-158684.

Change

Drop the lazy import and do a plain conditional import sysconfig in the two branches that actually need it. sysconfig is now loaded on Android/Cygwin and is neither loaded nor retained anywhere else. Behaviour is otherwise unchanged: the same two get_config_var() calls produce the same pythonapi on the same two platforms.

Tests

Added test.test_lazy_import.StdlibLazyImportTests.test_ctypes_does_not_leave_sysconfig_pending, which imports ctypes in a subprocess and asserts sysconfig is not left in sys.lazy_modules.

Results on 3.15.0rc3 with these files copied over its Lib/:

• new test against stock ctypes: FAIL - AssertionError: ['locale', 'pkgutil', 'sysconfig', 'traceback', 'warnings']
• new test against patched ctypes: ok
• test.test_lazy_import: 162 tests, OK
• test.test_ctypes: 675 tests, OK (40 skipped)
• ctypes.pythonapi.Py_GetVersion() still works

git diff --check is clean and the committed blobs are LF.

Note on validation scope: this machine has no MSVC toolchain (cl.exe), so I could not run a native CPython build or make patchcheck. The change is pure-Python and the suites above were run against a 3.15.0rc3 interpreter, not against a main build; the Android and Cygwin branches themselves could not be exercised here since this is Windows.

[python-cla-bot]

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[picnixz]

That's not the approriatae place for the test I think. It is likely in to be done in test_ctypes as we usually do for other modules. Avoid using LLM if the output is not properly revieweed.

[picnixz]

Isn't this lazy import still correct for other platforms? I don't think it's an issue per se though? many imports can be lazy because a single function uses them but if no one is using the function we still have a lazy pending import so I don't understand why this should be removed.

[picnixz]

I am not accepting this change as is. What we can accept is a test for lazy imports in test_ctypes. Plese look at other test modules to understand how to do it.

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