We recently dropped support for Python versions less than 3.5.
Since the README is also used as the project description on our
PyPI homepage, we need to be more explicit about what version of
Python our bindings require.
In order to keep all three packages (zulip, zulip_bots,
zulip_botserver) in the same repo, all package files must now
be nested one level deeper.
For instance, python-zulip-api/zulip_bots/zulip_bots/bots/, instead
of python-zulip-api/zulip_bots/bots/.
