Fix MANIFEST.in for setuptools

[mwtoews]

This resolves a few warnings from building this package, e.g. with uv build --sdist:

warning: no files found matching 'README.rst'
warning: no files found matching 'licence/licence_cache.json'
warning: no files found matching '*' under directory 'license/data'
warning: no files found matching '*.*' under directory 'license/data'
warning: no previously-included files found matching '.github'
warning: no previously-included files matching '__pycache__' found under directory '*'
warning: no previously-included files matching '*.py[co]' found under directory '*'

It also excludes environment.yaml, .gitattributes, pixi.lock, uv.lock and everything under .github/ in the resulting source distribution file.