cc @Rogdham (can't request review).

Best reviewed commit-by-commit.

This contains several clean-ups for the _zstd module.

• _compressionLevel_values is replaced with a single int constant, the default compression level
• zstd_version_info is initialised from the macros, removing version decomposition arithmetic
• remove the unused _ZSTD_CStreamSizes
• Replace _ZSTD_DStreamSizes with ZSTD_DStreamOutSize
• Remove unused _ZSTD_Config
• Remove zstd version string from error messages -- it is inconsistently used, and the version can be found from the module-level variables
• small refactoring of set_zstd_error()
• Inline the add_parameters function and use the existing PyModule_AddIntMacro rather than defining a local equivalent
• Remove the leading underscore from several constants in an already private module, use an explicit list of names for the re-exporting in compression.zstd

A

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