Added musllinux

[radarhere]

Resolves #226. Now that multi-build/multibuild#430 has been merged, we are able to build musllinux wheels.

I'm still waiting on multi-build/multibuild#465 though, so this PR temporarily diverges from the multibuild repository to my fork.

In order to accomodate macOS, manylinux and musllinux wheels, I have rearranged the jobs. Feel completely free to tell me you have a better idea for how to organise them.

As a note to self, it would be worth trying again with numpy after numpy/numpy#21200 makes it into a release.

[nulano]

Splitting the (now quite complicated) matrix into separate macOS/Linux files makes sense to me.

A previous PR #258 reorganized the build to avoid duplicating the steps. It seems like this PR undoes part of that. I'm not necessarily against, but if the steps are to be duplicated, it might make sense to move the if [[ "$TRAVIS_OS_NAME" == "osx" ]]; then brew remove ... part of build.sh to build-macos.yml as a separate step as this is specific to the macOS image provided by GitHub and not part of the actual build.

[nulano]

I thought musllinux is built using Alpine, not Ubuntu? Perhaps call this Linux wheels?

Suggested change

	name: Build Ubuntu wheels
	name: Build Linux wheels

[radarhere]

Fair call. Ok, I've pushed a commit

[hugovk]

Good stuff!

Some notes about optimising the build matrix, but these can definitely wait for followup PRs if you prefer, it's more important to get this in for Friday's release :)

---

macOS:

Most of the jobs are roughly the same, except for the PyPys, which are tacked on via include so appear last:

Maybe put the PyPys in the matrix, and exclude PyPy + arm64?

---

Linux:

Pretty good already, we have the slower PyPys first:

We could flip the platform order to put the 17m PyPy jobs before the 11m ones, and it doesn't make much difference for the other ones:

-        platform: [ "x86_64", "i686" ]
+        platform: [ "i686", "x86_64" ]

[radarhere]

Ok, I've switched macOS to exclude, and swapped the platform order for Linux.

[hugovk]

Looks good, thanks!

[radarhere]

multi-build/multibuild#465 has been merged, so I've created #279 to switch back to the main multibuild repository.