Mixin class for URLs that can be fetched over a proxy.
Get support for proxying and for URLs with user:pass@host setting.
Determine scheme, host and port for this connection taking
proxy data into account.
:return: tuple (scheme, host, port)
:rtype: tuple(string, string, int)
Check if self.host is in the $no_proxy ignore list.
Parse given proxy information and store parsed values.
Note that only http:// proxies are supported, both for ftp://
and http:// URLs.