Format with autopep8 and clean up code

[mbrukman]

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