ci(github-actions): add back 3.7 runner to pythonpackage

[Lee-W]

Description

• style: format the code through the latest black
• build(poetry): update black version for python 3.9 and upgrade least support python version to python 3.6.2
• ci(github-actions):
  • upgrade setup-python to v2
  • add back 3.7 runner to pythonpackage

Checklist

• Add test cases to all the changes you introduce
• Run ./script/format and ./script/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

3.7 runner added when running checkings

Steps to Test This Pull Request

send a pull request

Additional context

[codecov]

Codecov Report

Merging #456 (65645e0) into master (71aabe8) will increase coverage by 0.54%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #456      +/-   ##
==========================================
+ Coverage   97.37%   97.92%   +0.54%     
==========================================
  Files          39       39              
  Lines        1409     1395      -14     
==========================================
- Hits         1372     1366       -6     
+ Misses         37       29       -8     

Flag	Coverage Δ
unittests	97.92% <ø> (+0.54%)	⬆️

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

Impacted Files	Coverage Δ
commitizen/__main__.py	0.00% <0.00%> (ø)
commitizen/git.py	100.00% <0.00%> (+2.04%)	⬆️
commitizen/cli.py	97.61% <0.00%> (+2.16%)	⬆️
commitizen/cz/base.py	100.00% <0.00%> (+2.77%)	⬆️
commitizen/config/base_config.py	100.00% <0.00%> (+9.52%)	⬆️

---

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 d6a1e23...65645e0. Read the comment docs.