first a correction; there is one protocol where it make sense:
file
I would change:
...
elif scheme:
...
to
...
elif scheme and scheme != "file":
...


Second: no it is not a correct behaviour. Urlunsplit is supposed to create a valid url from a tuple created by urlsplit. :/// is not a valid url (except for the file protocol).

Third: rstrip could be simplified to rstrip(":/")