make string_to_datetime accept unicode

git-svn-id: https://svn.osgeo.org/grass/grass/trunk@74286 15284696-431f-4ddb-bdfa-cd5b030d7da7

lib/python/temporal/datetime_math.py

@@ -830,7 +830,7 @@ def string_to_datetime(time_string):
                  could not be converted
     """
 
-    if not isinstance(time_string, str):
+    if not isinstance(time_string, unicode) and not isinstance(time_string, str):
         return None
 
     time_object = check_datetime_string(time_string)