set requires-python to 3.8

[samuelcolvin]

I forgot before.

[codecov]

Codecov Report

Merging #94 (e9a685f) into main (5da8ed1) will not change coverage.
The diff coverage is n/a.

Additional details and impacted files

@@            Coverage Diff            @@
##              main       #94   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           12        12           
  Lines          814       814           
  Branches       150       150           
=========================================
  Hits           814       814           

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