Set min supported Python to `v3.9`

[rocco8773]

Set min supported Python to v3.9

• [x] GitHub Actions updated accordingly
• [x] Package configuration files updated accordingly

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 98.49%. Comparing base (4f25d2a) to head (b7f7748).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #132 +/- ## ======================================= Coverage 98.49% 98.49% ======================================= Files 47 47 Lines 3056 3057 +1 Branches 580 580 ======================================= + Hits 3010 3011 +1 Misses 44 44 Partials 2 2 ```

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