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