tests: migrate the pytype test to a shell script for easier CI processing

We can probably do more in the script (e.g. setup a venv and install pytype), but the CI file is currently installing pytype already. I didn't disable the *.t run because there's already a CI target for running pytype, so I assume swapping in this script is pretty simple for someone who understands the heptapod-ci.yml file. Depending on how the hookup with CI goes, we can drop the *.t test. But I'm waiting to see if the script gets CI specific tentacles, because I still want a convenient way to run it locally.