Recent versions of cmd2 have changed how the 'prog' attribute is
automatically set for ArgumentParser instances. As a result, we
are now seeing an unexpected 'build.py' artifact appearing in
the generated documentation.
Let's use an older release of cmd2, which retains the old expected
behavior. Use it specifically for building documentation.
Change-Id: Ifbad35adc5e9d3141acfd024d7dee2a25f1cb62e
Related: https://github.com/python-cmd2/cmd2/issues/1414
Related: OS#6776