pep8: Ignore line break before binary operator

PEP8 has switched its guidance [1] for where a line break should occur
in relation to a binary operator, so don't raise this warning for
new code and update the code base to follow the new style.

[1] https://www.python.org/dev/peps/pep-0008/#should-a-line-break-before-or-after-a-binary-operator

wa/utils/revent.py

@@ -155,8 +155,8 @@ class ReventRecording(object):
             else:  # not streaming
                 if not self._events:
                     self._duration = 0
-                self._duration = (self._events[-1].time -
-                                  self._events[0].time).total_seconds()
+                self._duration = (self._events[-1].time
+                                  - self._events[0].time).total_seconds()
         return self._duration
 
     @property