Merge #159: Fix struct.unpack receiving an integer instead of a byte

81ca8a3 Fix struct.unpack receiving an integer instead of a byte. stackoverflow.com/q/29299136/ (David)

Pull request description:

  Fix struct.unpack receiving an integer instead of a byte.

  Before I was receiving this error
  ```
    File "/python-bitcoinlib/venv/lib/python3.6/site-packages/python_bitcoinlib-0.9.0-py3.6.egg/bitcoin/core/script.py", line 684, in is_witness_scriptpubkey
      return 3 <= len(self) <= 42 and CScriptOp(struct.unpack('<b',self[0])[0]).is_small_int()
  TypeError: a bytes-like object is required, not 'int'
  ```

  Solved by getting the byte with self[0:1]. A bytearray of length 1 that is accepted by unpack.
  Check https://stackoverflow.com/q/29299136/

Tree-SHA512: 968d4b864baa820bb80f12b24031dbb9166aa3f46f612bb0f393eb5d445a869271591d100cebad528e3263079158e7a6bd7d565803b86e568b01223b55aa6f4a

Author: petertodd

bitcoin/core/script.py

@@ -680,7 +680,7 @@ def is_p2sh(self):
     def is_witness_scriptpubkey(self):
         """Returns true if this is a scriptpubkey signaling segregated witness
         data. """
-        return 3 <= len(self) <= 42 and CScriptOp(struct.unpack('<b',self[0])[0]).is_small_int()
+        return 3 <= len(self) <= 42 and CScriptOp(struct.unpack('<b',self[0:1])[0]).is_small_int()
 
     def witness_version(self):
         """Returns the witness version on [0,16]. """