Py3 compatibility fix: aliasing basestring to str

[pavel-kirienko]

This fix prevents the following error when downloading into a filelike object:

....
  File "./wd.py", line 21, in find_and_download_newest_file
    data = c.download(matching_item.name, bio)
  File "/usr/local/lib/python3.4/dist-packages/easywebdav/client.py", line 164, in download
    if isinstance(local_path_or_fileobj, basestring):
NameError: name 'basestring' is not defined