- Upgrade zstd source code from v1.5.0 to [v1.5.1](https://github.com/facebook/zstd/releases/tag/v1.5.1).
- Fix `ZstdFile.write()` / `train_dict()` / `finalize_dict()` may use wrong length for some buffer protocol objects.
- Two behavior changes:
- Setting `CParameter.nbWorkers` to `1` now means "1-thread multi-threaded mode", rather than "single-threaded mode".
- If the underlying zstd library doesn't support multi-threaded compression, no longer automatically fallback to "single-threaded mode", now raise a `ZstdError` exception.
- Add a module level variable [`zstd_support_multithread`](https://pyzstd.readthedocs.io/#zstd_support_multithread).
- Add a setup.py option `--avx2`, see [build options](https://pyzstd.readthedocs.io/#build-pyzstd).