Backwards incompatible changes: Support for Python 2 has been removed.
Note: This isn't a problem for Nixpkgs because
pythonPackages.cryptography is frozen at version 3.3.2.
Other important packaging changes: "Cryptography now incorporates Rust
code. Users building cryptography themselves will need to have the Rust
toolchain installed. Users who use an officially produced wheel will not
need to make any changes. The minimum supported Rust version is 1.45.0."
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