restore Python 3.9 compatibility

[FelixSchwarz]

| None is only valid for Python 3.10+.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 44.82%. Comparing base (23fca6d) to head (015527b).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #367      +/-   ##
==========================================
+ Coverage   43.59%   44.82%   +1.22%     
==========================================
  Files          31       31              
  Lines        4973     4973              
==========================================
+ Hits         2168     2229      +61     
+ Misses       2805     2744      -61     

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

[mar10]

Thanks