feat(serviceconfig): add overrides from Python repo metadata (#4186)

This populates new_issue_uri and documentation_uri when those values
aren't present in the service config (e.g. there's no publishing
section).

The tool itself *can* be deleted after this PR has been merged, but this
PR doesn't delete it in case it's useful when migrating other languages.
(Once we've migrated a few languages, it's less likely it'll be useful,
so we should definitely delete it then.)

Author: jskeet