revise importing on python 3.9 + bugfixes

[RonnyPfannschmidt]

reintroduces the shim in a simpler way

closes #1145
addresses python/importlib-metadata#515
closes #687

[Copilot]

Pull Request Overview

This PR revises the importing mechanism for Python 3.9 by removing the importlib-metadata dependency and implementing a simpler shim for handling entry points across different Python versions.

• Removes importlib-metadata dependency from project dependencies
• Implements version-specific entry_points functions to handle API differences between Python 3.9 and 3.10+

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
src/setuptools_scm/_entrypoints.py	Implements version-specific entry_points functions and removes importlib_metadata import
pyproject.toml	Removes importlib-metadata dependency for Python < 3.10

Comments suppressed due to low confidence (2)

src/setuptools_scm/_entrypoints.py:34

• The variable 'im' is not defined in the Python 3.9 branch. Since the top-level import was removed, this branch needs its own import statement for importlib_metadata.

        eps = im.entry_points()  # Returns dict

src/setuptools_scm/_entrypoints.py:43

• This import shadows the module-level 'im' import and creates inconsistent behavior. The function should use a different variable name or rely on the module-level import.

        import importlib.metadata as im

[Copilot]

This import will fail on Python 3.9 since importlib.metadata doesn't have the same API as importlib_metadata. The code should only import this for Python 3.10+ or handle the ImportError.