connection_pool: fix python 2 integer division

Add from __future__ import division to fix success_rate
and latency calculations returning 0 due to integer division.

connection_pool.py

@@ -1,5 +1,6 @@
 #!/usr/bin/env python2
 # -*- coding: utf-8 -*-
+from __future__ import division
 """Tor connection pooling for improved proxy testing throughput.
 
 This module provides connection pooling to reduce the overhead of