Update pypi and tests

What's Changed

• Integrated testing for multiple python versions by @JoranAngevaare in #80
• No more travis, pipy uploads via github actions by @JoranAngevaare in #86
  • Thanks to the great help of Joran Angevaare, we now use GitHub Actions and no longer use Travis for releases and testing.
  • pip install nestpy works again.
  • Should one want to recompile from source, you will still need to use git clone (for now).

New Contributors

• @JoranAngevaare made their first contribution in #80

Full Changelog: v1.5.2...v1.5.4