Avoid getting stuck in a loop

This prevents a case where we make a request to URL A, which 301s to B which
would then 301 back to A. Alternatively, for less simple schemes, this will
also prevent us from getting stuck in a loop, e.g., it will prevent the
following from causing an endless loop:

    A -> B -> C -> D -> E -> F --
    ^                             \
    |                             /
    ---<------------<----------<-

requests/sessions.py

@@ -535,8 +535,13 @@ class Session(SessionRedirectMixin):
         if not isinstance(request, PreparedRequest):
             raise ValueError('You can only send PreparedRequests.')
 
+        checked_urls = set()
         while request.url in self.redirect_cache:
-            request.url = self.redirect_cache.get(request.url)
+            checked_urls.add(request.url)
+            new_url = self.redirect_cache.get(request.url)
+            if new_url in checked_urls:
+                break
+            request.url = new_url
 
         # Set up variables needed for resolve_redirects and dispatching of hooks
         allow_redirects = kwargs.pop('allow_redirects', True)