Add Python 3.13 support

[vlogic]

Add Python 3.13 support

• Replace pkg_resources with importlib.metadata + packaging in
  liccheck/requirements.py and command_line.py. A Python 3.13 venv
  created the normal way has no setuptools, so pkg_resources was
  unimportable and liccheck crashed immediately after install.
  This removes the dependency on it entirely instead of papering
  over it with a setuptools pin.
• Keep pip._internal for requirements.txt parsing (still resolves
  fine against current pip; the only pip-free alternative library
  is stale and has an open bug against current packaging releases).
  Drop the dead pip<10 fallback import branches.
• Add packaging as a runtime dependency.
• Raise python_requires to >=3.8 and update classifiers (3.8-3.13).
• Add Python 3.12/3.13 to the CI matrix and tox envlist; drop the
  already-unsupported py35-py37 tox entries.
• Authorize Apache-2.0 in liccheck.ini for packaging's own license.
• Update tests: rewrite pkg_resources-based fixtures with
  importlib.metadata.PathDistribution, and adjust expected package
  counts for the new packaging dependency.

Verified on real Python 3.10-3.13 interpreters, including a clean
3.13 venv with no setuptools preinstalled.