Loading the ansible_mitogen Ansible plugins apparently doesn't follow the same
rules as importing a Python module. So sys.path manipulations in __init__.py
weren't fired when Ansible tried to load the plugins from a /custom/path that
wasn't already on sys.path.
This wasn't picked up by the test because CI always installs Mitogen as a
Python package (in a virtual env).
This reverses 6145508312.
Alex Willmer