This cleans up some pyflakes warnings that were found. There were only a

couple minor ones found in unit tests, and one entry that needed to be

added to the exclusion file.
This also fixes up run-pyflakes.py to exclude more files/paths by

default, and to support excluding subdirectories listed in

module_exclusions (in particular, reviewboard/htdocs/, which is handy if

you upload a Python file to a review request that has pyflakes errors).

PyFlakes no longer reported any errors or warnings.