ENH: Add GitPython and PyGithub external projects

Add external projects for GitPython, PyGithub, and their dependencies
(python-setuptools, python-gitdb, python-async, python-smmap). These are
required for full functionality of the SlicerWizard package.

These are only built if Python and extension manager support are both
enabled, and system Python is not used. Additionally, PyGithub is only
requested if Python SSL support is enabled.

Note that Python must be built with SSL support for PyGithub to be
usable.

Issue #3269

Co-authored-by: Jean-Christophe Fillion-Robin <jchris.fillionr@kitware.com>