Docs: switch coverage badge to Codecov

[ohmyfelix]

Summary

• replace the README Coveralls badge with Codecov
• keep the repository badges aligned with the current coverage reporting setup

Related

• Migrate remaining Coveralls usage to Codecov across contributte repos contributte#72

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.62%. Comparing base (f1fdf03) to head (196d842).
⚠️ Report is 13 commits behind head on master.

❗ There is a different number of reports uploaded between BASE (f1fdf03) and HEAD (196d842). Click for more details.

HEAD has 3 uploads less than BASE

Flag	BASE (f1fdf03)	HEAD (196d842)
	4	1

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #78      +/-   ##
==========================================
- Coverage   86.11%   80.62%   -5.49%     
==========================================
  Files           7        8       +1     
  Lines         144      191      +47     
==========================================
+ Hits          124      154      +30     
- Misses         20       37      +17     

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