Flake8 Fix

Added comment and fixed flake8 issue of missing blank line

Author: mikebiglan

rest_framework_jwt/compat.py

@@ -9,7 +9,9 @@
     class PasswordField(CharField):
         widget = widgets.PasswordInput
 else:
+    # Mid-file import to potentially prevent issues occuring for DRF pre 3.0.0
     import rest_framework.serializers
+
     class Serializer(rest_framework.serializers.Serializer):
         @property
         def object(self):