Enhancement: Update to Python 3.9 #256

Summary
Numpy 1.22 dropped support for Python 3.7. LD-Cool-P uses Pandas which depends on Numpy. LD-Cool-P uses Python 3.7 currently. Need to upgrade LD-Cool-P to use a newer version of Python. Python 3.10 is still too new so a migration to Python 3.9 is preferred. It's also a version that is already included in the redata-commons and ldcoolp-figshare.

Objectives
Migrate to Python 3.9

Proposal

• Update requirements.txt
• Update GH Actions (python-package.yml) to remove 3.7 test. Maybe add 3.10
• Update documentation (e.g., installation steps)

Testing notes
LD-Cool-P doesn't use Numpy directly. Need to verify that the current version of Pandas works ok with the new version of Numpy. Testing is to be done on the main LD-Cool-P functions

Additional notes

• Upgrade the production environment

Implemented in: TBD