Fixes flake8 error F401.
Most errors were fixed by running
autoflake -i -r --remove-all-unused-imports .
which also removes unnecessary 'pass' statements, some by hand-editing.
now a requests.Response is returned instead of the file-like object from urllib.
Fixed all usages of util.urlopen: it simplifies getting json, text encoding detection.
In particular feedcore (responsible for fetching feeds) is simplified.
This is a first pass and could benefit from better usage of the requests api
(Sessions for instance, to keep connection pools)
TODO: download.py