chore: update readme to check if response attribute of WebPushException object is not None (#168)

updated README.md and README.rst with check if response attribute of WebPushException is not None (thus is an instance of requests.Response)

this is because requests.Response.__bool__ method returns False if the status code is between 400 and 599 which prevents proper error handling if the Response turns out to be falsy

Author: 15bubbles

README.md

@@ -103,7 +103,7 @@ try:
 except WebPushException as ex:
     print("I'm sorry, Dave, but I can't do that: {}", repr(ex))
     # Mozilla returns additional information in the body of the response.
-    if ex.response and ex.response.json():
+    if ex.response is not None and ex.response.json():
         extra = ex.response.json()
         print("Remote service replied with a {}:{}, {}",
               extra.code,

README.rst

@@ -116,7 +116,7 @@ e.g. the output of:
    except WebPushException as ex:
        print("I'm sorry, Dave, but I can't do that: {}", repr(ex))
        # Mozilla returns additional information in the body of the response.
-       if ex.response and ex.response.json():
+       if ex.response is not None and ex.response.json():
            extra = ex.response.json()
            print("Remote service replied with a {}:{}, {}",
                  extra.code,