Fix installing stray files into site-packages (#14319)

Fix the `include` pattern in `pyproject.toml` not to install stray files
such as `CHANGELOG.rst`, `CONTRIBUTING.rst`, `docs` and `tests` straight
into site-packages.  Apparently Maturin did not install them before due
to a bug, but it was fixed in maturin 1.12.0, leading to the files being
suddenly installed.

Originally reported as https://bugs.gentoo.org/970090.

Signed-off-by: Michał Górny <mgorny@gentoo.org>