Drop python 3.8, support python 3.11

[codecov[bot]]

Codecov Report

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

Comparison is base (89824c4) 83.33% compared to head (72d599d) 83.34%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #343 +/- ## ========================================== + Coverage 83.33% 83.34% +0.01% ========================================== Files 161 161 Lines 12358 12358 ========================================== + Hits 10298 10300 +2 + Misses 2060 2058 -2 ``` | [Flag](https://app.codecov.io/gh/pySTEPS/pysteps/pull/343/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pySTEPS) | Coverage Δ | | |---|---|---| | [unit_tests](https://app.codecov.io/gh/pySTEPS/pysteps/pull/343/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pySTEPS) | `83.34% <ø> (+0.01%)` | :arrow_up: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pySTEPS#carryforward-flags-in-the-pull-request-comment) to find out more.

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