refactor: replace SemVer type literals by respective constants

[paduszyk]

Description

This is a small PR updating a few lines of code where SemVer types (major/minor/patch) are referred to as literal strings instead of constants defined in commitizen.defaults module.

I guess the changes made do not require tests.

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

[codecov]

Codecov Report

Patch coverage: 98.36% and project coverage change: +0.15% 🎉

Comparison is base (eb39f8b) 97.31% compared to head (756f088) 97.46%.
Report is 51 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #796      +/-   ##
==========================================
+ Coverage   97.31%   97.46%   +0.15%     
==========================================
  Files          42       42              
  Lines        2045     2090      +45     
==========================================
+ Hits         1990     2037      +47     
+ Misses         55       53       -2     

Flag	Coverage Δ
unittests	97.46% <98.36%> (+0.15%)	⬆️

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Files Changed	Coverage Δ
...en/cz/conventional_commits/conventional_commits.py	100.00% <ø> (ø)
commitizen/providers.py	97.52% <90.00%> (+0.22%)	⬆️
commitizen/commands/init.py	87.50% <94.44%> (+0.06%)	⬆️
commitizen/commands/bump.py	97.63% <96.55%> (-0.52%)	⬇️
commitizen/version_schemes.py	98.42% <98.42%> (ø)
commitizen/__version__.py	100.00% <100.00%> (ø)
commitizen/bump.py	100.00% <100.00%> (ø)
commitizen/changelog.py	100.00% <100.00%> (ø)
commitizen/changelog_parser.py	96.96% <100.00%> (+0.04%)	⬆️
commitizen/cli.py	94.20% <100.00%> (ø)
... and 14 more

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