Remove eventual trailing slash in Github URL
Before this patch, running the following command would fail to find the repository googlefonts/glyphsets/
(notice the trailing space) using the Github API:
$ py2dsp glyphsets --github https://github.com/googlefonts/glyphsets/ --verbose --profile dpt
...
github.GithubException.UnknownObjectException: 404 {"message": "Not Found", "documentation_url": "https://docs.github.com/rest"}
So one had to manually remove the trailing slash. This is now automatically done in the code.