Unify temporary directory allocation strategy in tests

[KmoM88]

Following the upstream unification in colcon/colcon-core#729, this PR refactors the test suite to use the modern tmp_path fixture (which provides a pathlib.Path object) instead of mixing multiple temporary directory strategies.

Specifically, this package previously used:

• Manual TemporaryDirectory management (in test/test_package_identification_python_setup_py.py)

This refactor:

1. Adds a test/conftest.py compatibility fixture for tmp_path (supporting older pytest versions, such as those on Enterprise Linux 8).
2. Updates test/test_package_identification_python_setup_py.py to accept and utilize the tmp_path fixture, removing the manual context manager.
3. Cleans up redundant global references in test/test_spell_check.py to prevent style guide validation errors on newer flake8 configurations.
4. Updates test/spell_check.words to include words introduced by the compatibility fixture (isdigit, itertools, linux, takewhile, tmpdir) and removes the now-unused tempfile.

Reference

• Aligns with the testing strategy established in: colcon/colcon-core#729

Specifics

colcon-python-setup-py relied entirely on manual TemporaryDirectory context manager blocks.

🤖 Assisted by Gemini

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 51.00%. Comparing base (9199b24) to head (eafeaa8).

Additional details and impacted files

@@           Coverage Diff           @@
##           master      #64   +/-   ##
=======================================
  Coverage   51.00%   51.00%           
=======================================
  Files           3        3           
  Lines         149      149           
  Branches       23       23           
=======================================
  Hits           76       76           
  Misses         67       67           
  Partials        6        6           

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