fix: correct singular/plural in macOS platform-tag warning

[nikolauspschuetz]

Problem

In calculate_macosx_platform_tag() the list of libraries that need a higher
deployment target is built and then immediately rebound to a joined string:

problematic_files = [k for k, v in versions_dict.items() if v > start_version]
problematic_files = "\n".join(problematic_files)
if len(problematic_files) == 1:
    files_form = "this file"
else:
    files_form = "these files"

By the time len(problematic_files) == 1 runs, problematic_files is a string,
so len() counts characters, not files. A single-file path is many characters
long, so the count is never 1 — the "this file" branch is effectively dead
and the warning always reads "these files", even when exactly one library is
at fault.

Fix

Compute files_form from the list length before joining.

Test

New tests/test_macosx_libfile.py (the module had no test file). It monkeypatches
extract_macosx_min_system_version so no real Mach-O binaries are needed, and is
parametrized over one vs. two .dylib files, asserting the warning says
"this file" vs. "these files" respectively. Fails before the fix (single
case), passes after.

docs/news.rst updated under a new **UNRELEASED** section.

_{for the record: fix found + patched with AI assistance; reviewed and tested by me.}

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 63.45%. Comparing base (21c4da4) to head (7c058a0).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #697      +/-   ##
==========================================
+ Coverage   57.22%   63.45%   +6.23%     
==========================================
  Files          15       15              
  Lines        1267     1267              
==========================================
+ Hits          725      804      +79     
+ Misses        542      463      -79     

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[henryiii]

Good fix, but I think I would rename the veritable instead.

[henryiii]

I would generally never recommend changing
the type of a variable. The string form should have a different name from the list form. Mypy has a check for this I believe. That would've protected from this problem.

[agronholm]

This was a large piece of contributed code which was mostly beyond my understanding, so this kind of thing was able to slip by unnoticed.

[nikolauspschuetz]

Good call — done in caa78fa. problematic_files now stays the list, and the joined string is a separate problematic_files_text, so the variable no longer changes type. Agreed a mypy no-redefinition check would have caught the original.