feat: pypy 3.8 & pip 21.3

[cibuildwheel-bot]

Update the versions of our dependencies.

PR generated by "Update dependencies" workflow.

Adds pip 21.3 and PyPy 7.3.7, along with pypy3.8. Updated the build identifier docs with two missing PyPy identifiers.

Closes #742

[henryiii]

Ohh, looks like PyPy is out. Manylinux in general is still stalled by travis issues pypa/manylinux#1207 and there's also an addition of the PyPy update in pypa/manylinux#1210. We'll need to add support for PyPy3.8 when updating PyPy. @YannickJadoul would you be interested in tackling the PyPy update (no rush until it lands in manylinux).

[henryiii]

There's a bin prefix warning, but I think the main issue here is that the "python" shortcut we create (IIRC) is now included in PyPy proper.

[henryiii]

PS: I don't think we should merge this until manylinux adds pip 21.3 and pypy 7.3.6, since we should ideally be somewhat constant across all platforms, and those are two big changes.

[henryiii]

PyPy 7.3.7 is out with fixes, but manylinux is still a bit stuck. I've done what I can, but someone needs to add GHA credentials for the ppc64le upload, or travis needs to fix their issues so it can move back there. Might be stuck until @mayeut returns.

[henryiii]

I think @mayeut will be back next week. Manylinux still needs authentication update to upload ppc from GHA.

[henryiii]

@mayeut or @joerick, does this look good to go? Will follow up with a few ideas before we release (like bumping PyPy's default to manylinux214)

[henryiii]

This will get wiped next automated update, so would like to get it in.

[joerick]

Looks good! Thanks @henryiii

[joerick]

Apologies for the delay in reviewing, been busy at work and with other projects so am a bit behind on my notifications. Let me know if we need a release soon and I can sort that.

[henryiii]

Okay, no problem. I just didn't want it to get wiped. I'm thinking #909 would be good to get into the next release if we want to do that (since there are already some PyPy changes). If #889 can be ready, that would be nice too, but probably not worth a delay over.