fix_authors.py wasn't including a closing parenthesis
The bot now always includes the person who opened the pull request in the authors list. Previously this was only done if all the commits had no login information, but if there were commits from someone else as well, it wasn't included.
Minor error message improvement when there are no release notes found
See https://github.com/sympy/sympy-bot/issues/47. Coverage may still be improvable in some places.
Also fixes some bugs: