Require Python 3.8, replace flake8 and isort with ruff

[codecov[bot]]

Codecov Report

Patch coverage: 92.85% and project coverage change: +0.61 :tada:

Comparison is base (e6a1a96) 94.60% compared to head (5dfe7d2) 95.21%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #190 +/- ## ========================================== + Coverage 94.60% 95.21% +0.61% ========================================== Files 12 12 Lines 482 481 -1 ========================================== + Hits 456 458 +2 + Misses 26 23 -3 ``` | [Impacted Files](https://app.codecov.io/gh/jodal/pykka/pull/190?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Stein+Magnus+Jodal) | Coverage Δ | | |---|---|---| | [src/pykka/\_envelope.py](https://app.codecov.io/gh/jodal/pykka/pull/190?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Stein+Magnus+Jodal#diff-c3JjL3B5a2thL19lbnZlbG9wZS5weQ==) | `100.00% <ø> (ø)` | | | [src/pykka/debug.py](https://app.codecov.io/gh/jodal/pykka/pull/190?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Stein+Magnus+Jodal#diff-c3JjL3B5a2thL2RlYnVnLnB5) | `0.00% <0.00%> (ø)` | | | [src/pykka/messages.py](https://app.codecov.io/gh/jodal/pykka/pull/190?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Stein+Magnus+Jodal#diff-c3JjL3B5a2thL21lc3NhZ2VzLnB5) | `100.00% <ø> (ø)` | | | [src/pykka/\_threading.py](https://app.codecov.io/gh/jodal/pykka/pull/190?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Stein+Magnus+Jodal#diff-c3JjL3B5a2thL190aHJlYWRpbmcucHk=) | `96.29% <75.00%> (ø)` | | | [src/pykka/\_\_init\_\_.py](https://app.codecov.io/gh/jodal/pykka/pull/190?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Stein+Magnus+Jodal#diff-c3JjL3B5a2thL19faW5pdF9fLnB5) | `87.50% <100.00%> (+8.55%)` | :arrow_up: | | [src/pykka/\_actor.py](https://app.codecov.io/gh/jodal/pykka/pull/190?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Stein+Magnus+Jodal#diff-c3JjL3B5a2thL19hY3Rvci5weQ==) | `95.27% <100.00%> (+0.92%)` | :arrow_up: | | [src/pykka/\_exceptions.py](https://app.codecov.io/gh/jodal/pykka/pull/190?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Stein+Magnus+Jodal#diff-c3JjL3B5a2thL19leGNlcHRpb25zLnB5) | `100.00% <100.00%> (ø)` | | | [src/pykka/\_future.py](https://app.codecov.io/gh/jodal/pykka/pull/190?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Stein+Magnus+Jodal#diff-c3JjL3B5a2thL19mdXR1cmUucHk=) | `100.00% <100.00%> (ø)` | | | [src/pykka/\_proxy.py](https://app.codecov.io/gh/jodal/pykka/pull/190?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Stein+Magnus+Jodal#diff-c3JjL3B5a2thL19wcm94eS5weQ==) | `99.01% <100.00%> (+<0.01%)` | :arrow_up: | | [src/pykka/\_ref.py](https://app.codecov.io/gh/jodal/pykka/pull/190?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Stein+Magnus+Jodal#diff-c3JjL3B5a2thL19yZWYucHk=) | `100.00% <100.00%> (ø)` | | | ... and [1 more](https://app.codecov.io/gh/jodal/pykka/pull/190?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Stein+Magnus+Jodal) | |

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