add extensions to the interface for Certificate
diff --git a/src/cryptography/x509.py b/src/cryptography/x509.py
index 71ba904..ec2092b 100644
--- a/src/cryptography/x509.py
+++ b/src/cryptography/x509.py
@@ -1125,6 +1125,12 @@
         in the certificate.
         """
 
+    @abc.abstractproperty
+    def extensions(self):
+        """
+        Returns an Extensions object.
+        """
+
     @abc.abstractmethod
     def __eq__(self, other):
         """