dev-python/sphinxcontrib-github-alt: new package.

dev-python/sphinxcontrib-github-alt: new package.

Link to GitHub issues, pull requests, commits and users for a particular
project.

Required to build the docs for jupyter_core.

It's called 'alt' because sphinxcontrib-github already exists. IPython &
Jupyter projects have been using the syntax defined in this extension for some
time before this made it into its own package.

Closes: https://bugs.gentoo.org/637990
Closes: https://bugs.gentoo.org/638200
Closes: https://github.com/gentoo/gentoo/pull/6246
Package-Manager: Portage-2.3.13, Repoman-2.3.3