exit map needs to be ported to Python 2

right now on check-01, we have the following Debian packages installed through puppet:

  # new exit scanner dependencies, see #33362
  ensure_packages([
    'build-essential',
    'curl',
    'gettext',
    'golang-go',
    'python-dateutil',
    'python-dnspython',
    'python-stem',
    'python3-stem',
  ], {
    ensure => installed,
  })
}

python-dateutil and python-dnspython are gone from Debian and during the major upgrade (#40692). i am assuming this would have broken the exitmap scaller, which depends on those. but i actually haven't checked!