I recommend adding a setup.py, but assuming that’s not an option, you can do what you ask by using two parts: a part to pull in python (and pip dependencies, if necessary), and a part to dump your python files where they need to go. That would look something like this:
# [...]
parts:
# Pull down python as well as any dependencies of the project
python:
plugin: python
python-version: python2
python-packages: [pip-dependency-1, pip-dependency-2]
# Dump the project's python files into the snap
my-python-stuff:
plugin: dump
source: .