Avoid using callable().

Callable() is not included in Python 3.1, so we shouldn't use it.

requests/models.py

@@ -9,6 +9,7 @@ This module contains the primary objects that power Requests.
 
 import os
 import socket
+import collections
 from datetime import datetime
 from io import BytesIO
 
@@ -467,10 +468,10 @@ class Request(object):
 
     def register_hook(self, event, hook):
         """Properly register a hook."""
-        if callable(hook):
+        if isinstance(hook, collections.Callable):
             self.hooks[event].append(hook)
         elif hasattr(hook, '__iter__'):
-            self.hooks[event].extend(h for h in hook if callable(h))
+            self.hooks[event].extend(h for h in hook if isinstance(h, collections.Callable))
 
     def deregister_hook(self, event, hook):
         """Deregister a previously registered hook.

tests/test_requests.py

@@ -11,6 +11,7 @@ import json
 import unittest
 import pickle
 import tempfile
+import collections
 
 import requests
 from requests.compat import str, StringIO
@@ -805,7 +806,7 @@ class RequestsTestSuite(TestSetup, TestBaseMixin, unittest.TestCase):
 
         def assert_hooks_are_callable(hooks):
             for h in hooks['args']:
-                self.assertTrue(callable(h))
+                self.assertTrue(isinstance(h, collections.Callable))
 
         hooks = [add_foo_header, add_bar_header]
         r = requests.models.Request()