🚇 Add CODECOV_TOKEN

[s-weigand]

The action codecov/codecov-action@v4 removed support for tokenless uploads so pushes to main failed upload coverage reports for the last 3 months.
However, uploads from PRs done by forks still work. so this issue can only be seen for pushes on main

Change summary

• 🚇 Add CODECOV_TOKEN

Checklist

• ✔️ Passing the tests (mandatory for all PR's)

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[github-actions]

👈 Launch a binder notebook on branch s-weigand/pyglotaran-extras/code-cov-token

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 44.77%. Comparing base (b880df9) to head (be7234a).
Report is 20 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #268   +/-   ##
=======================================
  Coverage   44.77%   44.77%           
=======================================
  Files          27       27           
  Lines        1043     1043           
  Branches      162      162           
=======================================
  Hits          467      467           
  Misses        568      568           
  Partials        8        8           

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