Use backports.zstd instead of pyzstd

[alexprengere]

https://pypi.org/project/backports.zstd/ looks to be an official backport of compression.zstd to earlier Python versions.
We should probably switch from pyzstd to use the same lib everywhere, but we cannot switch as-is because it looks like PyPy3 is not supported yet (see roadmap).
I created a PR #63 to test things.

Tagging @kurtmckee as the original author of #61

[kurtmckee]

PEP 784, which introduced zstd support, called out that the Python implementation uses pyzstd. The zstd code genealogy appears to be:

pyzstd --> CPython 3.14 stdlib --> backport

If the backport currently lacks implementation support already present in pyzstd, I don't yet see an advantage to migrating to the backport.