gh-132983: Remove pyzstd in identifiers

[Rogdham]

The _PyZstd_xxx extern functions were leftover artifacts of the move from pyzstd to cpython.

I have renamed them to the _zstd_xxx convention used for private functions.

• Issue: Implement PEP 784 - Adding Zstandard to the Python standard library #132983

[python-cla-bot]

All commit authors signed the Contributor License Agreement.

[Rogdham]

Thanks @emmatyping for the explanation about the context, pinging you for review as suggested 🤗

[emmatyping]

Looks good! Just want to make sure we make these static since they are local to their translation unit (file).

[emmatyping]

Test failure looks unrelated. Thanks for the PR!

[AA-Turner]

Note: in general we don't need to update the branch, it wastes CI resources. I tend to do so only sparingly.

[miss-islington-app]

Thanks @Rogdham for the PR, and @AA-Turner for merging it 🌮🎉.. I'm working now to backport this PR to: 3.14.
🐍🍒⛏🤖

[bedevere-app]

GH-133629 is a backport of this pull request to the 3.14 branch.

[emmatyping]

Note: in general we don't need to update the branch, it wastes CI resources. I tend to do so only sparingly.

I only did so to sort out CI failures which would block merge.

[AA-Turner]

Sorry! I missed that, thanks.