text: Title: Loudly deprecate old Python 2/3 compatibility layer and other deprecated utils
  Body: We "quietly" deprecated Python 2/3 compatibility layer in RF 5.0 (#4150) and `TRUE/FALSE_STRINGS` in RF 6.0 (#4500). Should deprecate them more loudly in RF 7.0. As discussed in #4150, deprecating constants is easiest with module `__getattr__`.