Drop support for Python 3.9

[LourensVeen]

Python 3.9 is out of support, so we should drop support for it, also because the new Ruff formatting introduces the use of | for unions, which was added in Python 3.10.

• Update metadata in pyproject.toml
• Update tox configuration
• Do we need to set the version for mypy, ruff?

[maarten-ic]

N.B. When dropping Python 3.9, we can also remove the importlib-metadata backport dependency and the associated import:

ymmsl-python/ymmsl/v0_2/resolver.py

Lines 21 to 24 in f4fc08e

	if sys.version_info < (3, 10):
	from importlib_metadata import EntryPoint, entry_points
	else:
	from importlib.metadata import EntryPoint, entry_points