preparations for musllinux

[liath]

This should get everything going once musllinux is available from pypa/manylinux#1135
I should caveat this PR by saying that afaik this is my first day touching anything in the python ecosystem so apologies if this is dumb lol

Also I kinda anticipate the pypy shuffling in cryptography-manylinux/Dockerfile-manylinux will not pass review but I included it in the off chance no one cares (why is pypy used in the pyca/cryptography builds?) or if any one wants to test building Alpine cryptography wheels with musllinux. Here's how I did that:

git clone git@github.com:pypa/manylinux
pushd manylinux
MANYLINUX_BUILD_FRONTEND=docker POLICY="musllinux_1_1" PLATFORM="x86_64" COMMIT_SHA="HEAD" ./build.sh
popd
git clone git@github.com:pyca/infra
pushd infra
docker build -f cryptography-manylinux/Dockerfile-manylinux -t ghcr.io/pyca/cryptography-musllinux_1_1:x86_64 --build-arg PYCA_RELEASE=musllinux_1_1_x86_64:HEAD .
pushd $(mktemp -d)
# essentially identical build to cryptography's wheel-builder.yml, except for pulling auditwheel@5 from git
curl -s 'https://gist.githubusercontent.com/Liath/83636c9001f65d6b8c03a2da3f4f5fa8/raw/99dfa9ed53db0e940bef24b0f2266994b77d2cf5/Dockerfile' -o Dockerfile
docker build -t pyca-cryptography-musllinux --build-arg CRYPTOGRAPHY_VERSION=3.4.8 .

[reaperhulk]

CI won’t pass until that docker image tag exists, but modulo my comments this generally looks fine. I look forward to seeing all of this land!

[liath]

Yay! I added the exclude block you mentioned to the cryptography PR too. :)