Make pmdc ready for Python3

A "blind run" (without any config or command-line parameters) of pmdc results in different output for Python2 (which was the only one at the time pmdc was written, I suppose) and Python3 (which soon will be the only supported version <4):

python-pmdc.mine/usr/sbin# python2 ./pmdc
Usage: pmdc <cache namespace> [<directory list>] [options]

pmdc: error: No cache provided
python-pmdc.mine/usr/sbin# python3 ./pmdc
  File "./pmdc", line 320
    parser.add_option("-m", "--output-file-mode", type="int", default=0644,
                                                                         ^
SyntaxError: invalid token

Can pmdc be maintained beyond the date where python2 becomes obsolete?