python: drop Python 2 support

Python 2.7 reached the end of its life on January 1st, 2020 [1]. Since
it would be a waste to ignore several Python 3.x features in master
discovery implementation, we decided to drop Python 2 support here.

Python 2 workaround cleanup activities are expected to be solved as
part of #212 solution.

1. https://www.python.org/doc/sunset-python-2/

Part of #196

Author: DifferentialOrange

tarantool/__init__.py

@@ -1,6 +1,10 @@
 # -*- coding: utf-8 -*-
 # pylint: disable=C0301,W0105,W0401,W0614
 
+import sys
+
+assert sys.version_info.major > 2, 'Connector supports Python 3 only'
+
 from tarantool.connection import Connection
 from tarantool.mesh_connection import MeshConnection
 from tarantool.const import (