Add compat modules for tomllib and importlib.metadata

[chrysle]

The conditional backports increased, thus I added two compatibility modules.

Contributor checklist

• Included tests for the changes.
• PR title is short, clear, and ready to be included in the user-facing changelog.

Maintainer checklist

• Verified one of these labels is present: backwards incompatible, feature, enhancement, deprecation, bug, dependency, docs or skip-changelog as they determine changelog listing.
• Assign the PR to an existing or new milestone for the target version (following Semantic Versioning).

[webknjaz]

Surfacing the errors from Sphinx (the CI logs get garbage-collected periodically, so it's best to have a copy in here to persist the context):

WARNING: autodoc: failed to import module 'cache' from module 'piptools'; the following exception was raised:
cannot import name 'PIP_VERSION' from partially initialized module 'piptools.utils' (most likely due to a circular import) (/home/runner/work/pip-tools/pip-tools/.tox/build-docs/lib/python3.13/site-packages/piptools/utils.py)
WARNING: autodoc: failed to import module 'repositories' from module 'piptools'; the following exception was raised:
cannot import name 'PIP_VERSION' from partially initialized module 'piptools.utils' (most likely due to a circular import) (/home/runner/work/pip-tools/pip-tools/.tox/build-docs/lib/python3.13/site-packages/piptools/utils.py)
WARNING: autodoc: failed to import module 'repositories.base' from module 'piptools'; the following exception was raised:
cannot import name 'PIP_VERSION' from partially initialized module 'piptools.utils' (most likely due to a circular import) (/home/runner/work/pip-tools/pip-tools/.tox/build-docs/lib/python3.13/site-packages/piptools/utils.py)
WARNING: autodoc: failed to import module 'repositories.local' from module 'piptools'; the following exception was raised:
cannot import name 'PIP_VERSION' from partially initialized module 'piptools.utils' (most likely due to a circular import) (/home/runner/work/pip-tools/pip-tools/.tox/build-docs/lib/python3.13/site-packages/piptools/utils.py)
WARNING: autodoc: failed to import module 'repositories.pypi' from module 'piptools'; the following exception was raised:
cannot import name 'PIP_VERSION' from partially initialized module 'piptools.utils' (most likely due to a circular import) (/home/runner/work/pip-tools/pip-tools/.tox/build-docs/lib/python3.13/site-packages/piptools/utils.py)
WARNING: autodoc: failed to import module 'resolver' from module 'piptools'; the following exception was raised:
cannot import name 'PIP_VERSION' from partially initialized module 'piptools.utils' (most likely due to a circular import) (/home/runner/work/pip-tools/pip-tools/.tox/build-docs/lib/python3.13/site-packages/piptools/utils.py)
/opt/hostedtoolcache/Python/3.13.5/x64/lib/python3.13/importlib/metadata/_meta.py:docstring of typing._overload_dummy:1: WARNING: py:class reference target not found: _T
/opt/hostedtoolcache/Python/3.13.5/x64/lib/python3.13/importlib/metadata/_meta.py:docstring of typing._overload_dummy:1: WARNING: py:class reference target not found: _T
/opt/hostedtoolcache/Python/3.13.5/x64/lib/python3.13/importlib/metadata/_meta.py:docstring of typing._overload_dummy:1: WARNING: py:class reference target not found: _T
/opt/hostedtoolcache/Python/3.13.5/x64/lib/python3.13/importlib/metadata/_meta.py:docstring of typing._overload_dummy:1: WARNING: py:class reference target not found: _T

So it seems like there are two problems:

• _T might need a temporary suppression
• a bunch of circular imports are likely a real problem and porting https://github.com/aio-libs/aiohttp/blob/91108c9/tests/test_circular_imports.py into this project's test suite should be able to help us