Updates the requirements on [pbkdf2](https://github.com/RustCrypto/password-hashes) to permit the latest version.
- [Release notes](https://github.com/RustCrypto/password-hashes/releases)
- [Commits](RustCrypto/password-hashes@scrypt-v0.6.0...pbkdf2-v0.7.1)

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