0.21.0

Backwards Compatibility Notes

• Support for Python 3.6 has been dropped. Python 3.7 is the minimum supported
  Python version.

Changes

• Bundled zstd library upgraded from 1.5.4 to 1.5.5.
• PyO3 Rust crate upgraded from 0.15 to 0.18.
• CI environment changed from Ubuntu 20.04 -> 22.04, Windows 2019 -> 2022,
  macOS 11 -> macOS 12.
• C types now use PyType_Spec and corresponding APIs. (#187) Contributed by
  Mike Hommey.