fix: Add a checkout step to make sure the python version can be determined from the pyproject.toml file

[nfelt14]

Proposed changes

This fixes a bug in the _reusable-package-testpypi.yml file where the python version can't be determined from the pyproject.toml file since the file doesn't exist.

Types of changes

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• Bug fix (non-breaking change which fixes an issue)

Checklist

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• Basic linting passes locally with my changes
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• I have updated the Changelog with a brief description of my changes

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (8ce9a17) to head (6030a8c).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main       #27   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files            4         4           
  Lines          189       189           
  Branches        39        39           
=========================================
  Hits           189       189           

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[github-actions]

Test Results (ubuntu)

path	passed	subtotal
tests/test_bump_version_in_files.py	5	5
tests/test_create_unique_testpypi_version.py	7	7
tests/test_find_unreleased_changelog_items.py	3	3
tests/test_update_development_dependencies.py	13	13
TOTAL	28	28

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