Only return the ReturnCode from _send_command

Now that I've implemented the protocol, I see how commands actually give
return values isn't how I thought it would be.  Accordingly,
_send_command now only returns the ReturnCode rather than wrapping it in
a list.  This allowed me to remove a few [0]s.

alpaca_isp/__init__.py

 
     def _send_command(self, cmd):
         """Send a command to the microcontroller, returning the result"""
-        rr = self._send_command_raw(cmd)
-        lr = [int(n) for n in rr.split()]
-        lr[0] = ReturnCode(lr[0])
-        if lr[0] != ReturnCode.CMD_SUCCESS:
-            raise ISPError(lr)
-        return lr
+        r = self._send_command_raw(cmd)
+        r = ReturnCode(int(r))
+        if r != ReturnCode.CMD_SUCCESS:
+            raise ISPError(r)
+        return r
 
     def enter_isp(self, delay=0.01):
         """Enter ISP mode by controlling the DTR (/RST) and RTS (/ISP) lines
                 "utf-8"))
         except ISPError as e:
             # Return a tuple for SECTOR_NOT_BLANK
-            if e.args[0][0] == ReturnCode.SECTOR_NOT_BLANK:
+            if e.args[0] == ReturnCode.SECTOR_NOT_BLANK:
                 offset = int(self._readline())
                 value = int(self._readline())
                 return (offset, value)
                 count).encode("utf-8"))
         except ISPError as e:
             # Return an offset for COMPARE_ERROR
-            if e.args[0][0] == ReturnCode.COMPARE_ERROR:
+            if e.args[0] == ReturnCode.COMPARE_ERROR:
                 return int(self._readline())
             raise