ENH: Re-enable PROPACK by default

[h-vetinari]

It's been a long story with PROPACK. There were a bunch of segfaults in the first scipy version that contained it, and ultimately it got disabled by default, because we weren't able to figure out the origin of the windows segfaults (main issue, though there were others...).

Now it seems though that the CI gods have shown some kindness and decided to let the PROPACK tests pass as of 1.10.x.

I discovered this after 1.10 was released, so the decision was to re-enable this for 1.11.

Fixes #15108

[rgommers]

It's still pretty thoroughly broken unfortunately with MKL and Accelerate, the ABI wrappers and build setup are buggy and require an overhaul. I fixed a couple of minor things last week, but there's quite a bit more to do. It's going to be a lot more prominent when macOS is going to default to Accelerate again. So I'd be inclined to leave this alone for 1.11.0.

[h-vetinari]

We could try enabling PROPACK by default for 1.11 in conda-forge, that way we can gauge the impact a bit, and if things go bad, it's easier to mark something as broken in conda-forge than on PyPI.

[rgommers]

That seems like a good approach.

[rgommers]

Converted to draft, will post on gh-15108 what is left to do before this can be merged.

[lucascolley]

will post on #15108 what is left to do before this can be merged

That comment is #15108 (comment)

[rgommers]

Continued in gh-19855, so closing this PR. Thanks @h-vetinari