python: Stop using basestring to test for strings

The base class basestring doesn't exist in Python 3. Use string_types
from six instead.

Change-Id: I7e84903fb7dd4a0af7ae4e9f4ec2e54338f212bb
Signed-off-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/15998
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Reviewed-by: Juha Jäykkä <juha.jaykka@arm.com>

src/python/m5/util/grammar.py

@@ -25,6 +25,7 @@
 # OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
 import os
+from six import string_types
 
 import ply.lex
 import ply.yacc
@@ -93,7 +94,7 @@ class Grammar(object):
             "'%s' object has no attribute '%s'" % (type(self), attr))
 
     def parse_string(self, data, source='<string>', debug=None, tracking=0):
-        if not isinstance(data, basestring):
+        if not isinstance(data, string_types):
             raise AttributeError(
                 "argument must be a string, was '%s'" % type(f))
 
@@ -113,7 +114,7 @@ class Grammar(object):
         return result
 
     def parse_file(self, f, **kwargs):
-        if isinstance(f, basestring):
+        if isinstance(f, string_types):
             source = f
             f = open(f, 'r')
         elif isinstance(f, file):