Feature: add support for .py files in py:percent format

[lukastk]

I submitted a PR here that adds support for .py files written using py:percent notation, essentially allowing for developing entirely using plain text files. I was inspired by this article, where the author also requested this feature.

To give a sample, it would look like this:

# %%
#|defaultexp my_module

# %%
#|export
def foo():
   print("Hello world")