Fork the pypa/manylinux repo and build slim images

[bastimeyer]

With the availability of native aarch64 CI runners on GH, forking pypa/manylinux and slimming down the builds is now an option without having to fall back to another free CI provider. GH's container registry can easily be used while everything's run here.

The manylinux images include Python builds of all supported Python versions, which bloats them up quite heavily. This is the main motivation for forking the build config.

In addition to that, OpenSSL is currently not built for manylinux_2_28 and the 1.1.1k builds from AlmaLinux 8 are included instead, as they pass py310+'s requirements.

Forking however means that upstream will need to be merged back regularly, because I won't maintain this myself. Merge conflicts shouldn't be a problem though unless we apply more modifications.

I'm not sure yet how I'm going to handle this repo if manylinux will be forked. Maybe it'll be replaced and its build config will be applied to the fork, or it'll build off of images of the fork (which would unnecessarily duplicate lots of data).