Improve Codecov configuration

[akx]

Codecov has started requiring a token some time back.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.09%. Comparing base (e0d1018) to head (b1af1fc).
Report is 17 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1103      +/-   ##
==========================================
+ Coverage   90.99%   91.09%   +0.09%     
==========================================
  Files          26       26              
  Lines        4444     4470      +26     
==========================================
+ Hits         4044     4072      +28     
+ Misses        400      398       -2     

Flag	Coverage Δ
macos-12-3.10	89.91% <ø> (?)
macos-12-3.11	89.91% <ø> (?)
macos-12-3.12	90.20% <ø> (?)
macos-12-3.8	89.83% <ø> (?)
macos-12-3.9	89.90% <ø> (?)
ubuntu-22.04-3.10	89.93% <ø> (?)
ubuntu-22.04-3.11	89.93% <ø> (?)
ubuntu-22.04-3.12	90.22% <ø> (?)
ubuntu-22.04-3.8	89.85% <ø> (?)
ubuntu-22.04-3.9	89.92% <ø> (?)
ubuntu-22.04-pypy3.10	89.93% <ø> (?)
windows-2022-3.10	90.27% <ø> (?)
windows-2022-3.11	90.27% <ø> (?)
windows-2022-3.12	90.57% <ø> (?)
windows-2022-3.8	90.20% <ø> (?)
windows-2022-3.9	90.27% <ø> (?)

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