If you run urlparse.urljoin() on a file: URL, the resulting URL 
has together with the wrong number of '/'s in it.

Properly formed, the URL (assuming no netloc) should have 
three slashes, so that it looks like "file:///...". The current 
code drops that down to one.

The error appears to be in a condition in urlunsplit(). It 
doesn't show up except in this one instance because the 
test is only run iff the scheme is in the list of those that can 
take a netloc and there is no netloc present in the URL. 
Apparently, this is pretty rare.

Patch attached that corrects the condition.