Doing a fallback test for // would look like
if scheme in uses_netloc and url[:2] == '//' or url[:2] == '//':

but this is equivalent to 
if url[:2] == '//':

i.e., an authority appears if and only if there is a // after the scheme.

This still allows a uses_netloc scheme to appear without //.

I have attached a patch against Python 2.7, svn revision 78212.  It adds s3 to netloc.