AIA hashing (#3911)

diff --git a/src/cryptography/x509/extensions.py b/src/cryptography/x509/extensions.py
index 9eff343..44d5be9 100644
--- a/src/cryptography/x509/extensions.py
+++ b/src/cryptography/x509/extensions.py
@@ -284,6 +284,9 @@
     def __getitem__(self, idx):
         return self._descriptions[idx]
 
+    def __hash__(self):
+        return hash(tuple(self._descriptions))
+
 
 class AccessDescription(object):
     def __init__(self, access_method, access_location):