Format with autopep8 and clean up code

Cleanups as follows (no functional changes):

* run `autopep8` on code to standardize formatting
* remove unused and commented-out imports
* use consistent 4-space indent (some code used 3 spaces)
* remove extra blank lines and trailing whitespace
* add missing spaces around `=`
* remove "# Imports" comments: it's obvious that they're imports, and imports
  are always at the top of the file
* order imports as per https://www.python.org/dev/peps/pep-0008/#imports
* sort imports when `autopep8` did not

Author: mbrukman