Add pertpy & decoupler to test matrix

[Zethson]

• Adds pertpy and decoupler to the test matrix
• Allows for optional extras to be specified per package
• Adds a README

[flying-sheep]

Feel free to reject!

Why would we? it's good to add more here!

Ideally, these tests would be skipped if they're not available. It's a TODO of mine.

let's wait for that, there's little utility in adding tests that can't succeed yet

[Zethson]

@flying-sheep added an extras matrix to have the DE extra available for pertpy. This fails as expected because scverse/PyDESeq2#387 is not yet resolved. The pertpy tests have run through without errors without the extra but I think it's nice that we cover pydeseq2 through this extra. Lots of functionality of pertpy is gated behind it.

@PauBadiaM the decoupler errors should be looked into

[PauBadiaM]

@Zethson, the error was caused because in a test I used scipy.sparse.coo_matrix to test if decoupler works with multiple sparse formats. This worked for older anndata versions. I have changed it to scipy.sparse.csc_matrix to make it compatible.

[Zethson]

@PauBadiaM the failures are coming from Statsmodels. We need to pin scipy to <1.16 in decoupler, please.