This installs PL/Python (specifically plpython3u), enables it in the
database, and creates a function id36_to_id that calls the Python
function with the same name inside the tildes.lib.id module. This will
enable doing queries similar to this, when I have a topic's ID36 from
the site:

SELECT * FROM topics WHERE topic_id = id36_to_id('asdf');

The fact that this was possible to set up without having to port the
id36_to_id logic to a different language is blowing my mind a little.
There are some really interesting possibilities from being able to
import all of the Python code into the database itself.