if debian/pyversions doesn't exist (as in -5, closes: 588820, thanks to
Vincent Legout)

  - print error message if file cannot be opened (closes:
    521508, thanks to Carl Chenet for the original patch)
  - print nice error message if computed set of supported versions is empty
    (closes: 583392)

    supported versions and to make it the default version

modified:
  debian/changelog
  debian/pyversions.py

    handled by py3versions in python3-defaults

modified:
  debian/changelog
  debian/pyversions.py