Preserve compatible metadata versions in convert

[itscloud0]

Summary

Preserve the source metadata version during wheel convert when it is already compatible, instead of always rewriting converted metadata to 2.4.

Closes #643.

Problem

On current main, wheel convert rewrites converted PKG-INFO metadata to Metadata-Version: 2.4 unconditionally.

That means converting an egg with Metadata-Version: 2.1 produces a wheel whose METADATA advertises 2.4, even when the converted fields only require the older format. The issue report shows that this breaks downstream repositories such as Artifactory instances that still reject metadata newer than 2.3.

The packaging spec also says build tools may emit the lowest metadata version that supports the fields they write.

Solution

• keep the source metadata version when it is already high enough for the converted fields
• only bump older source metadata to 1.2, which is the minimum version needed for the generated Requires-Dist and Project-URL fields
• update the convert expectations and add direct regression tests for both 1.1 -> 1.2 and 2.1 -> 2.1

Verification

• .venv/bin/pytest -q
• .venv/bin/pytest tests/commands/test_convert.py -q
• .venv/bin/ruff check src/wheel/_commands/convert.py tests/commands/test_convert.py
• PYTHONPATH=src .venv/bin/python - <<'PY' ... probe confirming 1.1 -> 1.2 and 2.1 -> 2.1

[codecov]

Codecov Report

❌ Patch coverage is 81.81818% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 55.60%. Comparing base (33650c6) to head (acee32b).

Files with missing lines	Patch %	Lines
src/wheel/_commands/convert.py	81.81%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #690      +/-   ##
==========================================
+ Coverage   55.39%   55.60%   +0.20%     
==========================================
  Files          15       15              
  Lines        1213     1223      +10     
==========================================
+ Hits          672      680       +8     
- Misses        541      543       +2     

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[agronholm]

So is it that Artifactory still doesn't support metadata 2.4?

[agronholm]

All this touches is the metadata version number. Have you checked that the output metadata can still be compatible with all those previous metadata versions? Surely there have been significant changes over time.

[henryiii]

I'm only familiar with the 2.1-2.6 range, but generally you do not ever want to auto-update the version if it was produced as an older version. For example, 2.2 make a change that you cannot auto-upgrade: it adds meaning to missing values that wasn't there before. I don't know about the pre-2 to 2 transition though (I think it's less defined, IIRC around 2.1 was the first strongly defined version? Maybe misremembering. I'm sure you could ask an LLM).

[agronholm]

From my cursory LLM chat, it seems like keeping the metadata version should be safe, so this PR is fine.

[agronholm]

Aside from the questions I asked, this also needs a changelog entry in docs/news.rst.

[agronholm]

LGTM!