Upgraded to Python 3.8 and 3.9

2021.10.14

Moving up to Python 3.8 and 3.9

Streamlined and improved the documentation.

Streamlined the documentation, adding a description for the plug-in.
Also tidied up the description for PyPI, which is used by the new installer, and added keywords to help with searches.

Updated for compatibility with the new system classes.

Merge pull request #2 from molssi-seamm/system2

Updated for compatibility with the new system classes

Internal: moving to GitHub Actions for CI/CD

No change in functionality, but making sure that everything works using GitHub Action for continuous integration (CI) and deployment (CD)