Preserve importable ModuleType subclass references on CPython

[CAOShurong]

Fixes #397.

The module reducer uses exact-type dispatch, so a ModuleType subclass used as a
global can prevent a function from being pickled by value. On CPython, let an
importable subclass with default reduction hooks use the existing module
reference reducer. Exact custom dispatch and custom reduction hooks retain their
precedence, including modules whose custom attribute access must not be inspected.
Unregistered and explicitly by-value subclasses remain unsupported rather than
being flattened into ordinary modules.

Regressions cover protocols 2/4/5, direct modules and function globals, custom
dispatch/hooks, and fresh-process import success and missing-module failure.

Validation on Windows:

• CPython 3.13.1 full suite twice: 341 passed, 10 skipped, 2 expected xfails.
• All 278 existing test outcomes unchanged; 75 additional cases pass. On the
  unchanged runtime, the new cases have 9 failures and 66 passes.
• Focused cases repeated on CPython 3.13.1 and 3.11.16, including non-editable
  wheel installs; the original dump/load reproducer and reducer-control matrix pass.
• Black, Ruff, wheel/sdist build, Twine, dependency and diff checks pass.

Native ROOT, Dask/Optuna integration, PyPy and other platforms were not tested.