maintain-github-mirror: fix dependency name
maintain-github-mirror: fix dependency name

The script needs the 'github' module, but this is provided by the
'PyGithub' library on Pypi, not the 'github' library.

Change-Id: I7c6d4b30a11d38e060fb4e6b2cdf29301f6f03e3

openstack-infra/project-config,openstack-infra/project-config