Add an A-normal form transformer for Python code to pyct.

The purpose of A-normal form is to assign every intermediate value to
an explicit variable, so that downstream transformations have those
variables to associate information with.
https://en.wikipedia.org/wiki/A-normal_form

This transformer is mostly complete, but there are a few corner cases
with room for improvement (notably constructs that only appear in Python 3).

PiperOrigin-RevId: 206619935