Add Codecov token to CI

[ararslan]

Coverage hasn't been properly submitted in a long time. The issue as reported by the CI job is that we need a token, so I've added one in the repository settings and this commit just tells the action to use it.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 39.03%. Comparing base (c1e8462) to head (983326b).
Report is 18 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #318   +/-   ##
=======================================
  Coverage   39.02%   39.03%           
=======================================
  Files         233      233           
  Lines        6144     6151    +7     
  Branches     1608     1608           
=======================================
+ Hits         2398     2401    +3     
- Misses       3393     3397    +4     
  Partials      353      353           

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