Refactor swh.lister.github.utils to swh.core.github.utils
This also:
- extracts pytest fixtures needed to cover the GitHubSession code.
- avoid mutating the inputted credentials passed in constructors
- rework test docstrings to "it should" format for more concise description
Related to swh-lister#4232 (closed)
TODO in other diffs:
- D7840: Move pytest fixtures into a runtime pytest module (to be reusable in swh.lister)
- !273 (closed): Branch the use of GitHubSession in the new function to compute the canonical github urls
Migrated from D7840 (view on Phabricator)