Compatability with latest cryptography versions (45+)

[benrutter]

It looks like asyncssh doesn't work with versions of the cryptography library from 45 onwards.

It looks like, attempting to connect via asyncssh to an SFTP server with cryptography versioned 45+ will throw an ValueError: Invalid DH Parameters expection on this line:

      30    self._pn = dh.DHParameterNumbers(b, g)
->    31    self._priv_key = self._pn.parameters().generate_private_key()
      32

For context, I'm using via sshspec (fsspec) but I don't think it's tied to the connection method there.

(Have to admit that I've tried and failed to find the release notes for cryptography so I'm not sure what changed in version 45)

Expected behavior would probably be, either pinning to pre-45, or supporting both (or only supporting 45+ and pinning to post that version)