remove autopep8 dependency

[edublancas]

we use autopep8 to normalize code so whitespace changes do not mark task as outdated:

ploomber/src/ploomber/codediffer.py

Line 83 in de6d14b

code = autopep8.fix_code(code)

however, these requires us to add autopep8 as a dependency. we don't really need to use autopep8, as long as we have a preprocessor that normalizes code, it should work. one approach we can take is to strip whitespace in every line. autopep8 does more than this but I think it's worth switching. are there any common cases I'm ignoring?