type hint update for Python 3.9 and later

[sinisaos]

Improved type hints for Python 3.9 and later. This can be done in other Piccolo repositories as well. See comment

[codecov-commenter]

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.02%. Comparing base (1157c12) to head (524c044).
Report is 43 commits behind head on master.

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #450      +/-   ##
==========================================
+ Coverage   93.42%   94.02%   +0.60%     
==========================================
  Files           5        5              
  Lines         365      402      +37     
==========================================
+ Hits          341      378      +37     
  Misses         24       24              

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[dantownsend]

@sinisaos Looks great - thanks! 👍