support pixi environments

[keewis]

• closes support pixi #30
• also closes better error messages for policy schema failures #34 and closes error msg if version is missing #35

This adds initial support for pixi environments.

It does so by manually parsing the manifest file (which needs to be passed explicitly to the action), extracting the features from the given environment (the name of the env is passed as pixi:<env-name>) and merging them into a single list of pins.

There's still a lot to be done before it can be merged:

• PyPI dependencies are ignored without warning warns now
• the tests are lacking / non-existent
• no documentation

cc @VeckoTheGecko

[keewis]

I believe with this we should have a working version (it's still far from where I'd like it to be regarding tests, but this is already much better than before).

I'll keep this open for a couple of days to double check there's nothing wrong with it and will then merge and release.

[VeckoTheGecko]

will do some more testing later

[VeckoTheGecko]

I've been testing with the xarray manifest, and all the minimum envs there have the 'test' feature, which has

array-api-strict = "<2.4"

as a dep, but this errors with

ValueError: Unknown version format: <2.4

since this function doesn't support "<" as a specifier

[VeckoTheGecko]

Actually, I realised that the env I was looking at was the wrong one (test-py311-bare-min-and-scipy when it should be test-py311-min-versions - the latter which has array-api-strict pinned to something else). So I don't think this is a problem

[keewis]

it actually is a problem (but not with this action, see below): array-api-strict comes from the test feature and is included in test-py311-bare-minimum, which we definitely want to check.

However, I would argue that array-api-strict is not part of the test deps (like pytest or hypothesis), but rather a dependency that should be part of the extras feature. Which means the pin would not be included in the min versions envs, side-stepping this issue.

[VeckoTheGecko]

Finished the testing I wanted to do with the xarray manifest - didn't run into any more issues (except those listed above) :)