Fix missing basestring on python3

[umlaeute]

python3 lacks the 'basestring' class, instead all strings are unicode and of type 'str'.
this patch makes easywebdav py3-able

[ghost]

+1
would be cool if you apply this fix

[afsneto]

Just insert at the beginning of the code (after import statements):

try:
    unicode = unicode
except NameError:
    # 'unicode' is undefined, must be Python 3
    str = str
    unicode = str
    bytes = bytes
    basestring = (str,bytes)
else:
    # 'unicode' exists, must be Python 2
    str = str
    unicode = unicode
    bytes = str
    basestring = basestring