PythonUtil: Fix usage of reduce() for Python 3

Backport to 1.10.7; see #980

direct/src/showbase/PythonUtil.py

@@ -44,6 +44,7 @@ from panda3d.core import ConfigVariableBool
 
 if sys.version_info >= (3, 0):
     import builtins
+    from functools import reduce
     xrange = range
 else:
     import __builtin__ as builtins