Update tests

[yannikschaelte]

• use codecov action (previous implementation led to strange report merging problems. not an issue here, but does not hurt this way I guess)
• test on python 3.8
• temporarily fixate python-libcombine==0.2.3_1 until https://github.com/sbmlteam/libCombine/issues/29 is resolved

[codecov[bot]]

Codecov Report

Merging #435 into develop will not change coverage. The diff coverage is n/a.

@@           Coverage Diff            @@
##           develop     #435   +/-   ##
========================================
  Coverage    77.91%   77.91%           
========================================
  Files           22       22           
  Lines         2205     2205           
  Branches       527      527           
========================================
  Hits          1718     1718           
  Misses         362      362           
  Partials       125      125           

---

Continue to review full report at Codecov.

Legend - Click here to learn more Δ = absolute <relative> (impact), ø = not affected, ? = missing data Powered by Codecov. Last update b050558...8b48de1. Read the comment docs.