Improve health of this package (#1409)

* added flake8 and flake8-import-order to travis for py27

* fixed a test that fails from time to time depending on an order of a dict

* flake8 tweaks for the entire codebase excluding tests

mongoengine/base/datastructures.py

@@ -1,5 +1,5 @@
-import weakref
 import itertools
+import weakref
 
 from mongoengine.common import _import_class
 from mongoengine.errors import DoesNotExist, MultipleObjectsReturned
@@ -199,8 +199,9 @@ class BaseList(list):
     def _mark_as_changed(self, key=None):
         if hasattr(self._instance, '_mark_as_changed'):
             if key:
-                self._instance._mark_as_changed('%s.%s' % (self._name, 
-                    key % len(self)))
+                self._instance._mark_as_changed(
+                    '%s.%s' % (self._name, key % len(self))
+                )
             else:
                 self._instance._mark_as_changed(self._name)