handle PEP 639 License-Expression in test metadata

[nolith]

Problem

test_resolve_distribution_information asserts dist.metadata["License"] == "MIT" against the installed pytest package. This fails when pytest is built with PEP 639 metadata (setuptools >= 77), because the License header no longer exists — it is replaced by License-Expression.

Why upstream CI doesn't catch it

The dev dependencies pin pytest==8.4.1, which was built with older setuptools and still has the legacy License: MIT header in its METADATA. The test only fails with pytest >= 9.0, which requires setuptools>=77 and emits PEP 639 metadata (License-Expression: MIT with no License field).

Any environment that doesn't use the pinned pytest version (e.g. nixpkgs, conda, other distros) hits this failure.

Fix

Use .get() to check License-Expression first, then fall back to License. This handles both old-style and PEP 639 metadata:

license_value = dist.metadata.get("License-Expression") or dist.metadata.get("License")
assert license_value == "MIT"

Using .get() instead of [] also avoids the DeprecationWarning from importlib.metadata about implicit None returns on missing keys, which will become a KeyError in a future Python version.

[nolith]

I didn't go as far as upgrading pytest, but on a local test I can confirm the entire test suite works with pytest-9.0.2

. .venv/bin/activate && python -m pytest
============================= test session starts ==============================
platform darwin -- Python 3.13.12, pytest-9.0.2, pluggy-1.6.0
rootdir: /Users/nolith/src/nix/plux
configfile: pyproject.toml
collected 47 items
tests/cli/test_discover.py ......                                        [ 12%]
tests/cli/test_entrypoints.py .........                                  [ 31%]
tests/cli/test_hatchling.py ...                                          [ 38%]
tests/runtime/test_cache.py ..                                           [ 42%]
tests/runtime/test_resolve.py .                                          [ 44%]
tests/test_discovery.py ..                                               [ 48%]
tests/test_entrypoint.py ...                                             [ 55%]
tests/test_function_plugin.py ...                                        [ 61%]
tests/test_listener.py ....                                              [ 70%]
tests/test_manager.py .............                                      [ 97%]
tests/test_metadata.py .                                                 [100%]
============================= 47 passed in 15.21s ==============================

[thrau]

nice catch Alessio @nolith, thanks for the contribution!

this change looks good to me. i would also have been ok with just removing the assert. but this makes it much clearer. i was vaguely aware that the license metadata field had been changed, but hadn't connected the dots to plux, so thanks!