Changelog:
 - Added multi-threaded scanning support.
 - Added built-in Windows manual page (see `-m`/`--manual`); credit [Adam Russell](https://github.com/thecliguy).
 - Added version check for OpenSSH user enumeration (CVE-2018-15473).
 - Added deprecation note to host key types based on SHA-1.
 - Added extra warnings for SSHv1.
 - Added built-in hardened OpenSSH v8.5 policy.
 - Upgraded warnings to failures for host key types based on SHA-1.
 - Fixed crash when receiving unexpected response during host key test.
 - Fixed hang against older Cisco devices during host key test & gex test.
 - Fixed improper termination while scanning multiple targets when one target returns an error.
 - Dropped support for Python 3.5 (which reached EOL in Sept. 2020).
 - Added 1 new key exchange: `sntrup761x25519-sha512@openssh.com`.