Add pip-wheel-metadata to Python.gitignore

PEP-517 has resulted in some updates to the Python
build process. As a result, a new directory called
pip-wheel-metadata is created on fresh builds.
This PR adds this directory into the Python default
gitignore. See
https://github.com/pypa/pip/blob/e5f4bbb7ddff87f48f2b5815513e4211ccdde83a/src/pip/_internal/req/req_install.py#L568