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
e5f4bbb7dd/src/pip/_internal/req/req_install.py (L568)

Python.gitignore

@@ -20,6 +20,7 @@ parts/
 sdist/
 var/
 wheels/
+pip-wheel-metadata/
 share/python-wheels/
 *.egg-info/
 .installed.cfg