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.
Given an extracted download of mitogen-2.2.tar.gz, with strategy_plugins
pointing into it, if an old version of the package was pip-installed,
then the old pip-installed package would be imported and override
whatever came from the tarball.
Instead, modify sys.path before attempting any import. This still isn't
perfect, but it's better.
Ansible's PluginLoader makes up bullshit when it imports a module
(mostly because it has to make up something), therefore we ended up with
duplicate copies of ansible_mitogen loaded: one under
ansible.plugins.*.mitogen, and one under the canonical namespace.
Which broke isinstance().
Alex Willmer