split codetools.github_token() out of codetools.login_github()
split codetools.github_token() out of codetools.login_github()

This is to allow the bulk of login_github() to be reused for auth using
`pygithub` instead of `github3.py`.

lsst-sqre/sqre-codekit,lsst-sqre/sqre-codekit