Support Python 3.10 in CI/local testing

[dtrodrigues]

Description

Adds CI support for Python 3.10 and bump pytest to a version which is compatible with 3.10.

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

Steps to Test This Pull Request

Additional context

pytest 6.2.5+ is needed for Python 3.10: https://docs.pytest.org/en/6.2.x/changelog.html#pytest-6-2-5-2021-08-29

[codecov]

Codecov Report

Merging #465 (1f37002) into master (1216a9e) will increase coverage by 0.07%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #465      +/-   ##
==========================================
+ Coverage   97.96%   98.04%   +0.07%     
==========================================
  Files          39       39              
  Lines        1424     1431       +7     
==========================================
+ Hits         1395     1403       +8     
+ Misses         29       28       -1     

Flag	Coverage Δ
unittests	98.04% <100.00%> (+0.07%)	⬆️

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Impacted Files	Coverage Δ
commitizen/__version__.py	100.00% <100.00%> (ø)
commitizen/commands/check.py	100.00% <100.00%> (ø)
commitizen/changelog.py	96.47% <0.00%> (+0.70%)	⬆️

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