* Added the capability to defer invoking passphrase callback until
an encrypted private key is actually used in a signing operation,
rather than triggering the callback when keys are loaded. This
will only work when a public key is provided with an encrypted
private key either explicitly or as part of the key format (such
as in OpenSSH's private key format).
* Improved handling of KeyboardInterrupt and task cancellation in
SCP. Thanks go to Viktor Kertesz for reporting this issue and
helping to understand the behavior in various versions of Python.
* Fixed the env option to support mappings other than dict. Thanks
go to Boris Pavlovic for reporting this issue.
* Fixed a potential race condition in SSHForwarder cleanup. Thanks
go to GitHub user misa-hase for reporting this issue and helping
to test the fix.