Only the Mitogen unit tests will run against CentOS 5, providing atleast some
Python 2.4test coverage. There is no version of Ansible that supports Python
2.4 that is also supported by Mitogen 0.3.
The SSH key exchange argument is to persuade newer SSH clients to talk with
such an old SSH server.
See https://www.openssh.org/legacy.html
Although these are synonyms in Python 2.x, when using MyPy to typecheck
code use of file() causes spurious errors.
This commit also serves as one small step to Python 3.x compatibility,
since 3.x removes the file() builtin.
Alex Willmer