Issue #129: Run tests against all Python versions 3.6 - 3.11.

[DavidHuebner]

• Run tests against Python 3.9, 3.10 and 3.11
• Adapted package tags
• Building against Ubuntu 20.04 as ubuntu-latest is currently being migrated to ubuntu-22 and is still unstable (https://github.com/actions/setup-python/issues/555)

[codecov-commenter]

Codecov Report

Merging #130 (e2db635) into dev (25d5ab9) will increase coverage by 0.00%. The diff coverage is n/a.

@@           Coverage Diff           @@
##              dev     #130   +/-   ##
=======================================
  Coverage   92.16%   92.17%           
=======================================
  Files          16       16           
  Lines        2144     2146    +2     
=======================================
+ Hits         1976     1978    +2     
  Misses        168      168           

Impacted Files	Coverage Δ
setup.py	0.00% <ø> (ø)
averbis/highlighting/_utils.py	93.54% <0.00%> (+0.44%)	:arrow_up:

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