urllib.parse doesn't seem to round-trip file URI's containing multiple leading slashes.  For example, this--

    import urllib.parse

    def round_trip(url):
        parsed = urllib.parse.urlsplit(url)
        new_url = urllib.parse.urlunsplit(parsed)
        print(f'{url} [{parsed}]\n{new_url}')
        print('ROUNDTRIP: {}\n'.format(url == new_url))

    for i in range(4):
        round_trip('file://{}root/a'.format(i * '/'))

results in--

    file://root/a [SplitResult(scheme='file', netloc='root', path='/a', query='', fragment='')]
    file://root/a
    ROUNDTRIP: True

    file:///root/a [SplitResult(scheme='file', netloc='', path='/root/a', query='', fragment='')]
    file:///root/a
    ROUNDTRIP: True

    file:////root/a [SplitResult(scheme='file', netloc='', path='//root/a', query='', fragment='')]
    file://root/a
    ROUNDTRIP: False

    file://///root/a [SplitResult(scheme='file', netloc='', path='///root/a', query='', fragment='')]
    file:///root/a
    ROUNDTRIP: False

URI's of the form file:////<host>/<share>/<path> occur, for example, when one wants to git-clone a UNC path on Windows:
https://stackoverflow.com/a/2520121/262819

Here is where CPython defines urlunsplit():
https://github.com/python/cpython/blob/4e11c461ed39085b8495a35c9367b46d8a0d306d/Lib/urllib/parse.py#L465-L482
(The '//' special-casing seems to occur in this line here:
https://github.com/python/cpython/blob/4e11c461ed39085b8495a35c9367b46d8a0d306d/Lib/urllib/parse.py#L473 )
 
And here is where the round-tripping is tested:
https://github.com/python/cpython/blob/4e11c461ed39085b8495a35c9367b46d8a0d306d/Lib/test/test_urlparse.py#L156
(Three initial leading slashes is tested, but not the problem case of four or more.)