Increase the usage of augmented assignment statements

[elfring]

:eyes: Some source code analysis tools can help to find opportunities for improving software components. :thought_balloon: I propose to increase the usage of augmented assignment statements accordingly.

diff --git a/stripe/stripe_object.py b/stripe/stripe_object.py
index 8dbe49f..498b0ae 100644
--- a/stripe/stripe_object.py
+++ b/stripe/stripe_object.py
@@ -208,14 +208,14 @@ class StripeObject(dict):
         # updating a customer, where there is no persistent card
         # parameter.  Mark those values which don't persist as transient
         if partial:
-            self._unsaved_values = self._unsaved_values - set(values)
+            self._unsaved_values -= set(values)
         else:
             removed = set(self.keys()) - set(values)
-            self._transient_values = self._transient_values | removed
+            self._transient_values |= removed
             self._unsaved_values = set()
             self.clear()
 
-        self._transient_values = self._transient_values - set(values)
+        self._transient_values -= set(values)
 
         for k, v in six.iteritems(values):
             super(StripeObject, self).__setitem__(