Apply isort & flake8 (#312)

* Update isort settings
* Apply import sorting (isort) on all files in nyaa/
* Fixed Flake8 errors in nyaa/ (see PR for list)
* Add isort to lint.sh and requirements.txt

nyaa/backend.py

@@ -1,16 +1,14 @@
-import flask
-from nyaa import app, db
-from nyaa import models, forms
-from nyaa import bencode, utils
-
-import os
-
 import json
-from werkzeug import secure_filename
-from collections import OrderedDict
-from orderedset import OrderedSet
+import os
 from ipaddress import ip_address
 
+import flask
+from werkzeug import secure_filename
+
+from orderedset import OrderedSet
+
+from nyaa import app, db, models, utils
+
 
 @utils.cached_function
 def get_category_id_map():