Actually use Python 3.8.

[toolness]

This fixes #922 and ensures that we always use Python 3.8. It also adds some tests to ensure that we're always using the expected version of Python going forward.

To do

• Add a dev/prod test that makes sure the Pipfile specifies the right version of Python.
• Uncomment test case lines commented-out in Put online rental history users on a TextIt follow-up campaign. #921.
• Consider adding another field to the healthcheck (or extended healthcheck) that includes the current Python version. (I ended up doing this; I had some concerns about security but since everything in this project is open source, it seemed moot since attackers can easily tell what version of Python we're using by looking at our source code.)