Easify list by (default) conditions

I found that the current `pygithub3.services.issues.Issue.list_by_repo`
implementations does not work. Overriding the default parameters did not
seem to work for me.

This PR removes the default parameters. The default parameters don't
need to be passed.
Merge pull request #38 from Kijewski/dont-post-defaults

Easify list by (default) conditions