python: Replace deprecated repr syntax

Change-Id: I5f9538cf2ca5ee17c51e7c5388d3aef363fcfa54
Signed-off-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/15989
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>

src/python/m5/util/grammar.py

@@ -134,5 +134,5 @@ class Grammar(object):
 
     def t_error(self, t):
         msg = "Illegal character %s @ %d:%d" % \
-            (`t.value[0]`, t.lineno, t.lexpos)
+            (repr(t.value[0]), t.lineno, t.lexpos)
         raise ParseError(msg, t)