use basestring rather then (str, unicode) in isinstance

Author: y-p

pandas/tseries/tools.py

@@ -36,7 +36,7 @@ def _infer(a, b):
 
 
 def _maybe_get_tz(tz):
-    if isinstance(tz, (str, unicode)):
+    if isinstance(tz, basestring):
         import pytz
         tz = pytz.timezone(tz)
     if com.is_integer(tz):