Remove dev tools from production dependencies

[patdhlk]

Found in: #121

Problem

pre-commit>=3.5, pytest>=7.0, and packaging>=20.0 are listed in [project.dependencies] in pyproject.toml. This means anyone who pip install sphinx-simplepdf gets these installed as transitive dependencies.

• pre-commit and pytest are dev/test tools and should only be in [dependency-groups].dev
• packaging has a comment saying it replaces pkg_resources.parse_version but is not imported anywhere in the codebase — it should be removed entirely unless code that uses it is added

Action needed

Move pre-commit and pytest to [dependency-groups].dev. Remove packaging unless it's actually used.