Bump minimum Python version from 3.8 to 3.9

[pont-us]

Fixes #920.

Checklist:

• ~[ ] Add unit tests and/or doctests in docstrings~ n/a
• ~[ ] Add docstrings and API docs for any new/modified user-facing classes and functions~ n/a
• ~[ ] New/modified features documented in docs/source/*~ n/a
• [x] Changes documented in CHANGES.md
• [x] GitHub CI passes
• [x] AppVeyor CI passes
• [x] Test coverage remains or increases (target 100%)

[codecov[bot]]

Codecov Report

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

Comparison is base (1107099) 93.43% compared to head (bb140fb) 93.43%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #921 +/- ## ======================================= Coverage 93.43% 93.43% ======================================= Files 453 453 Lines 35518 35518 ======================================= + Hits 33186 33188 +2 + Misses 2332 2330 -2 ```

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