test(changelog): fix datetime mocking error

[Lee-W]

Description

as title

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

Steps to Test This Pull Request

Additional context

[codecov]

Codecov Report

Base: 98.25% // Head: 98.36% // Increases project coverage by +0.10% 🎉

Coverage data is based on head (836a94a) compared to base (764861f).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #563      +/-   ##
==========================================
+ Coverage   98.25%   98.36%   +0.10%     
==========================================
  Files          39       39              
  Lines        1551     1590      +39     
==========================================
+ Hits         1524     1564      +40     
+ Misses         27       26       -1     

Flag	Coverage Δ
unittests	98.36% <100.00%> (+0.10%)	⬆️

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/bump.py	100.00% <100.00%> (ø)
commitizen/cmd.py	100.00% <100.00%> (ø)
commitizen/commands/changelog.py	96.62% <100.00%> (+0.03%)	⬆️
commitizen/commands/commit.py	98.43% <100.00%> (+0.02%)	⬆️
commitizen/commands/init.py	91.66% <100.00%> (ø)
commitizen/config/json_config.py	100.00% <100.00%> (ø)
commitizen/config/yaml_config.py	100.00% <100.00%> (ø)
commitizen/defaults.py	100.00% <100.00%> (ø)
commitizen/exceptions.py	100.00% <100.00%> (ø)
... and 2 more

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