Work around a Python 2.6.1 bug with urlparse

On Python versions 2.6.1 and earlier, urlparse.urlparse parses URLs with unknown schemes incorrectly, like so:

>>> urlparse.urlparse("ssh://user@host/path")
ParseResult(scheme='ssh', netloc='', path='//user@host/path', params='', query='', fragment='')

Work around this in URL.__new__.

I'll probably add a corresponding workaround to URL.__init__ later, but as only known schemes can be accessed anyway, this isn't nearly as high of a priority.