Fix this the same way gdb/python/lib/gdb/printing.py handles it.
gdb/Changelog:
2017-03-17  Jonah Graham  <jonah@kichwacoders.com>
	PR gdb/19637
	* python/lib/gdb/printer/bound_registers.py: Add support for
	Python 3.
+2017-03-17  Jonah Graham  <jonah@kichwacoders.com>
+
+       PR gdb/19637
+       * python/lib/gdb/printer/bound_registers.py: Add support for
+       Python 3.
+
 2017-03-16  Andreas Arnez  <arnez@linux.vnet.ibm.com>
 
        * dwarf2loc.c (indirect_synthetic_pointer): Get data type of
 
 
 import gdb.printing
 
+if sys.version_info[0] > 2:
+    # Python 3 removed basestring and long
+    basestring = str
+    long = int
+
 class MpxBound128Printer:
     """Adds size field to a mpx __gdb_builtin_type_bound128 type."""