Workaround for lack of xrange function in python3 (old-style configuration files)

The xrange function has been replaced by range in python3. (The old python2 range function has been dropped.) This merge request uses from past.builtins import xrange to allow xrange to be used in python3, while maintaining the behaviour in python 2. This fix is only applied to some old-style configuration code to allow some unit tests to succeed and should not be used in CA-based configuration code.