There are two zstd modules on pypi:

    https://pypi.org/project/zstd/
    https://pypi.org/project/zstandard/
    
The first one is too simple.

The second one is powerful, but has too many APIs:
    ZstdCompressorIterator
    ZstdDecompressorIterator
    ZstdCompressionReader
    ZstdCompressionWriter
    ZstdCompressionChunkerIterator
    (multi-thread compression)

IMO these are not necessary for stdlib.

In addition, it needs to add something, such as the `max_length` parameter, and a `ZstdFile` class that can be integrated with the tarfile module. These workloads are not big.

I looked at the zstd API, it's a bit simpler than lzma/bz2/zlib. If spend a month, should be able to make a zstd module for stdlib. Then discuss the detailed API on Python-Ideas.
 
I once wanted to do this job, but it seems my time does not allow it. If anyone wants to do this work, please reply here.

FYI, Python 3.10 schedule:
    3.10.0 beta 1: 2021-05-03 (No new features beyond this point.)