hexverify: Fix handling unicode strings on Python 3.2

We were getting this error on Python 3.2:

    ERROR: runTest (Crypto.SelfTest.Hash.common.MACSelfTest)
    CMAC #17: NIST SP 800 38B D.7 Example 17
    ----------------------------------------------------------------------
    Traceback (most recent call last):
      File "build/lib.linux-x86_64-3.2/Crypto/SelfTest/Hash/common.py", line 199, in runTest
        self.assertRaises(ValueError, h.hexverify, "4556")
      File "/home/dwon/py/pythons/python3.2/lib/python3.2/unittest/case.py", line 557, in assertRaises
        callableObj(*args, **kwargs)
      File "build/lib.linux-x86_64-3.2/Crypto/Hash/CMAC.py", line 323, in hexverify
        self.verify(unhexlify(hex_mac_tag))
    TypeError: 'str' does not support the buffer interface