Lots of nice little dev-related changes:
- Run PHPUnit v9.5, if possible
- Add
void returns to unit tests (#54 & #57)
- Start testing against PHP 8.0 (#55)
- Update Psalm to v4.7
- Update PHP-CS-Fixer to v3.0
- Update Doctrine Coding Standard to v9 (#57)
- Switch from TravisCI to GitHub Actions since travis-ci.org is shutting down (#56)
