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