Add compatibility helper for older Pytest versions

[cottsay]

The tmp_path fixture was introduced in Pytest 3.9. Include an explicit definition of that fixture for compatibility with those older Pytest versions. When we're ready to drop support, we can just remove the fixture.

We could test this in GitHub Actions but it would be pretty contrived. I think it would be OK to carry this as a compatibility extension without explicitly testing it in CI, but I'm open to hearing differing opinions.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 51.82%. Comparing base (78cb048) to head (c0586cd).

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #47   +/-   ##
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  Coverage   51.82%   51.82%           
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  Files          20       20           
  Lines        2678     2678           
  Branches      546      546           
=======================================
  Hits         1388     1388           
  Misses        971      971           
  Partials      319      319           

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[j-rivero]

Seems like pytest.version_tuple is not exported in pytest 3.9–6.x, so getattr(pytest, "version_tuple", ()) < (3, 9) still evaluates true. This triggers an unfortunate chain of problems:

1. On pytest 3.9.3 and 6.2.5, pytest.version_tuple is not exported.
2. getattr(..., ()) returns ().
3. In Python, () < (3, 9) is True.
4. The compatibility fixture is defined even though pytest already provides tmp_path.

Checked locally with uv:

/tmp ❯ uv pip install "pytest<7.0"
Resolved 7 packages in 80ms
Prepared 4 packages in 84ms
Installed 4 packages in 4ms
 + attrs==26.1.0
 + py==1.11.0
 + pytest==6.2.5
 + toml==0.10.2
/tmp ❯ python3
Python 3.10.20 (main, Mar 20 2026, 00:33:12) [Clang 22.1.1 ] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import pytest
>>> getattr(pytest, 'version_tuple', ()) < (3, 9)
True
>>> getattr(pytest, 'version_tuple', ()) 
()

[cottsay]

😞 that's a bummer. I'll have a closer look. Thanks!