chore(deps): unpin cryptography

[vivodi]

cryptography is only a transitive dependency and doesn’t need to be managed directly by us.

Under default conditions, an appropriate version will be installed automatically.

Pinning it instead unnecessarily increases dependency complexity and raises confusion as to why it’s being explicitly fixed.

[vivodi]

If you @oz123 believe that using an older version of cryptography is insecure, you should pin it in the PyOpenSSL project instead (and if you consider it necessary, you’re welcome to submit a PR there).

The pytest-localftpserver project does not import cryptography directly, so pinning it here does not align with best practices in software engineering.