Fix test_trailing_slash_directory_metadata for Python 3.15

[KRRT7]

Summary

Python 3.15 alpha tightened importlib.metadata to raise MetadataNotFound eagerly when no metadata files exist on disk. The test used a fake non-existent path (/path/to/foo.egg-info) to verify trailing-slash handling — this worked on older Pythons where metadata access was lazy, but now fails on 3.15.

Replace the fake path with a real temp directory containing a minimal PKG-INFO file. The test still verifies the same behavior (name parsing and trailing-slash normalization).

Fixes #13901.

Test plan

• nox -s test-3.15 -- tests/unit/metadata/test_metadata.py::test_trailing_slash_directory_metadata — 2 passed (Python 3.15.0a8)
• nox -s test-3.12 -- tests/unit/metadata/test_metadata.py::test_trailing_slash_directory_metadata — 2 passed (Python 3.12.3)

[notatallshaw]

Do we know what changed upstream to break this?

[KRRT7]

CPython 3.15.0a8 intentionally changed importlib.metadata to raise MetadataNotFound when no metadata file exists, where previously it returned an empty PackageMetadata. From the 3.15.0a8 changelog:

In importlib.metadata, when a distribution file is corrupt and there is no metadata file, calls to Distribution.metadata() (including implicit calls from other properties like .name and .requires) will now raise a MetadataNotFound Exception. This allows callers to distinguish between missing metadata and a degenerate (empty) metadata.

The test was using a fake non-existent path, which worked before because accessing .metadata on a missing path silently returned empty metadata. Now it raises.

There's also a related regression tracked upstream: python/cpython#143387

[KRRT7]

The upstream fix is python/cpython#146234 (merged). The original change that introduced MetadataNotFound came from python/importlib_metadata#532. Other projects hit the same thing — e.g., rpm-software-management/rpmlint#1488 did the same fix (add real PKG-INFO to egg-info test mocks).

[ichard26]

@sbidoul this should fix failing CI on main. I haven't reviewed this yet, FWIW.

[sbidoul]

Looks good, thanks!