long deprecated string.split removed in python 3.7

string.split deprecated in python 2, and removed from python 3.7 (fedora 29's default python3), see trace below, attached patch fixes the bug.

--- Start running command STATUS at 10:23:20.103 ---
Traceback (innermost last):
  File "/usr/bin/duplicity", line 1699, in <module>
    with_tempdir(main)
  File "/usr/bin/duplicity", line 1685, in with_tempdir
    fn()
  File "/usr/bin/duplicity", line 1518, in main
    action = commandline.ProcessCommandLine(sys.argv[1:])
  File "/usr/lib64/python3.7/site-packages/duplicity/commandline.py", line 1189, in ProcessCommandLine
    globals.backend = backend.get_backend(args[0])
  File "/usr/lib64/python3.7/site-packages/duplicity/backend.py", line 228, in get_backend
    obj = get_backend_object(url_string)
  File "/usr/lib64/python3.7/site-packages/duplicity/backend.py", line 214, in get_backend_object
    return factory(pu)
  File "/usr/lib64/python3.7/site-packages/duplicity/backends/pydrivebackend.py", line 100, in __init__
    folder_names = string.split(parsed_url.path, u'/')
 AttributeError: module 'string' has no attribute 'split'

10:23:22.417 Task 'STATUS' failed with exit code '30'.