chore: improve package classifiers

[jenstroeger]

Description

Fixes #548

Checklist

• Add test cases to all the changes you introduce
• Run ./scripts/format and ./scripts/test locally to ensure this change passes linter check and test
• Test the changes on the local machine manually
• Update the documentation for the changes

Expected behavior

n/a

Steps to Test This Pull Request

n/a

Additional context

n/a

[codecov]

Codecov Report

Merging #553 (764861f) into master (ed636ba) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #553      +/-   ##
==========================================
- Coverage   98.26%   98.25%   -0.01%     
==========================================
  Files          39       39              
  Lines        1556     1551       -5     
==========================================
- Hits         1529     1524       -5     
  Misses         27       27              

Flag	Coverage Δ
unittests	98.25% <100.00%> (-0.01%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
commitizen/__version__.py	100.00% <100.00%> (ø)
commitizen/cmd.py	100.00% <100.00%> (ø)
commitizen/git.py	100.00% <100.00%> (ø)

Help us with your feedback. Take ten seconds to tell us how you rate us. Have a feature suggestion? Share it here.

[woile]

Thanks!