remove ipv6 brackets from hostname for httplib

fixes #1568, makes ipv6 actually work like this:

 a = requests.get('http://[2001:718:1:4::2]/')

It might be good idea to use urrlib3 newer than
https://github.com/shazow/urllib3/commit/208ba74c796116bafc95d46dfcf4325449643ab6
as it fixes the problem too.