source_address parameter to HTTPConnection.__init__ was introduced only in 2.7 so this would break 2.6 compatibility.
See https://github.com/pasha-r/urllib3/commit/518d0c324add739705717fb6864620d224dae62a - source_address is optional in overriding class, call to super's __init__ goes without it and after the call I set self.source_address which is all __init__ of super class does :)