Increase miniumum version of Python from 3.7 to something more modern

[mjordan]

https://devguide.python.org/versions/ shows Python 3.7 has passed end of life. We should increase the minimum required to something more modern.

Here are the libraries that Workbench currently uses, and their stated minimum requirements:

• ruamel.yaml - Python >=3
• Requests - Python >=3.7
• Requests-Cache - Python >=3.7
• progress_bar - Python 3+
• openpyxl - Python >=3.6
• unidecode - Python >=3.5
• edtf-validate - Python >=3.7
• rich - Python >=3.7.0
• typing-extensions - only required by 3.7 or lower.

So barring any specific incompatibilities with versions of Python greater than 3.7, looks like we don't have any libraries that lock us to 3.7.

[mjordan]

Related - #834.

[mjordan]

As of #849 Workbench only officially supports Python 3.9 and higher. There is a "Python 3.8 EOL" release for anyone who cannot upgrade.