python3: use string byte literal

Fix Tarantool connection liveness and box-py/call.test.py test.

Part of #20

Co-authored-by: Alexander Turenko <alexander.turenko@tarantool.org>

Author: ligurio

lib/tarantool_connection.py

@@ -146,7 +146,7 @@ def opt_reconnect(self):
         """
         try:
             if not self.is_connected or self.socket.recv(
-                    1, socket.MSG_DONTWAIT | socket.MSG_PEEK) == '':
+                    1, socket.MSG_DONTWAIT | socket.MSG_PEEK) == b'':
                 self.reconnect()
         except socket.error as e:
             if e.errno == errno.EAGAIN: