Now the built `_awscrt.abi3.so` is marked as compatible with shadow
stacks (SHSTK), as assembly files from aws-lc are no longer used.

* Dropped the comment about aws-cli-v2, which is no longer in [extra]
* Use git checksums

That may not be supported by upstream in near future

aws-c-http changes ABI (adding a new field to a public struct) without
updating soname [1], resulting crashes in awscrt [2]. It's not the
first time for silent ABI changes. I assume AWS guarantees no ABI
stability for AWS CRT libs for now, so moving to bundled libs, which
come with pinned dependencies, makes more sense.

[1] https://github.com/awslabs/aws-c-http/commit/c7350090b8712d9a6fd66a7d5d45056c87879f26
[2] https://bugs.archlinux.org/task/78870

See: https://bugs.archlinux.org/task/76618

Although aws-cli-v2 wants awscrt <= 0.14.0, 0.15.0+ will be needed for
testing on Python 3.11

python-awscrt: new package, dependency for upcoming aws-cli-v2 and optional dependency for several other packages