test_accept_enforce_host_keys fails with check_host_keys='accept' on recent /usr/bin/ssh versions

[moreati]

On Ubuntu 20.04 ssh has a new third option when it prompts for an unknown host key:

± ssh gist.github.com
The authenticity of host 'gist.github.com (64:ff9b::8c52:7903)' can't be established.
RSA key fingerprint is SHA256:nThbg6kXUpJWGl7E1IGOCspRomTxdCARLviKw6E5SY8.
Are you sure you want to continue connecting (yes/no/[fingerprint])?

This string is no longer matched by HOSTKEY_REQ_PATTERN, causing SshTest.test_accept_enforce_host_keys() to fail thus

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ERROR: test_accept_enforce_host_keys (ssh_test.SshTest)
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Traceback (most recent call last):
  File "/home/alex/src/mitogen/tests/ssh_test.py", line 161, in test_accept_enforce_host_keys
    check_host_keys='accept',
  File "/home/alex/src/mitogen/tests/testlib.py", line 544, in docker_ssh
    return self.router.ssh(**kwargs)
  File "/home/alex/src/mitogen/mitogen/parent.py", line 2502, in ssh
    return self.connect(u'ssh', **kwargs)
  File "/home/alex/src/mitogen/mitogen/parent.py", line 2444, in connect
    return self._connect(klass, **mitogen.core.Kwargs(kwargs))
  File "/home/alex/src/mitogen/mitogen/parent.py", line 2426, in _connect
    raise mitogen.core.StreamError(self.connection_timeout_msg)
mitogen.core.StreamError: Connection timed out.

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