First PYPI release

Changes:

• CI was changed from Travis to GitHub Actions:
  • ubuntu-latest, ubuntu-18.04
  • python: 3.6, 3.7, 3.8, 3.9
• First PYPI package is created: https://pypi.org/project/twittertennis/

First release

This release represents the state of the twittertennis package before refactoring it in 2020 for better compatibility with pytorch geometric.