CI: Fix and improve configuration

[amotl]

About

Investigate and fix test suite failures on two PRs submitted by Dependabot.

• Chore(deps): Bump codecov/codecov-action from 4 to 5 #197
• Chore(deps-dev): Update twine requirement from <6 to <7 #198

Details

• Don't make the test matrix fail fast
• Update to PyPy 3.9
• Fix configuration for codecov-action, it needs a secret now

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.29%. Comparing base (7303ec2) to head (81b5698).
Report is 59 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #199      +/-   ##
==========================================
- Coverage   94.44%   92.29%   -2.16%     
==========================================
  Files          26       27       +1     
  Lines        1711     1816     +105     
==========================================
+ Hits         1616     1676      +60     
- Misses         95      140      +45     

Flag	Coverage Δ
unittests	92.29% <ø> (?)

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