Remove dependency on `pkg_resources`

[sbidoul]

setuptools 82 removed pkg_resources. #125 helps with that. uv however does not select setuptools-odoo 3.3.2 (see #126 (comment)) so although workaround exist (see #126 (comment)), we may want to stop relying on pkg_resources.

We have these build-time uses:

• pkg_resources.parse_version in git_postversion.py: we can replace with packaging.version
• pkg_resources.resource_string in base_addons.py: we can replace with stdlib importlib.resources (need to check compatible python versions)

We use it in tests (can be replaced in a second time, or not at all because we'll still need pkg_resources in test addons):

• pkg_resources.find_distributions: we can replace with stdlib importlib.metadata (need to check compatible python versions)

At runtime on older Odoo versions we have __import__('pkg_resources').declare_namespace(__name__) in __init__.py but these Odoo versions need pkg_resources at runtime too so that is not an issue.

All in all, still a bit of work, though.