We still create a Python 2 virtualenv for thumbor but that’s
separate (/srv/zulip-thumbor-venv from
scripts/lib/create-thumbor-venv).
Signed-off-by: Anders Kaseorg <andersk@mit.edu>
The testing section is more appropriate, since it's fundamentally part
of our CI system.
While we're at it, fix the fact that we were linking to GitHub, not
ReadTheDocs, in the run-mypy output.