Move utils.calc_msg_key into auth_key (cyclic imports py3.4)

telethon/crypto/auth_key.py

@@ -4,7 +4,6 @@ This module holds the AuthKey class.
 import struct
 from hashlib import sha1
 
-from .. import helpers as utils
 from ..extensions import BinaryReader
 
 
@@ -36,4 +35,6 @@ class AuthKey:
         """
         new_nonce = new_nonce.to_bytes(32, 'little', signed=True)
         data = new_nonce + struct.pack('<BQ', number, self.aux_hash)
-        return utils.calc_msg_key(data)
+
+        # Calculates the message key from the given data
+        return sha1(data).digest()[4:20]