[jaster]

  > cat test.py 
  import sys
  if sys.version_info.major < 3:
      raise Exception("this script requires python version >= 3")
  
  print(f"test")
  
  > python2 test.py
    File "test.py", line 5
      print(f"test")
                  ^
  SyntaxError: invalid syntax

Doesn't seem that simple

[int_19h]

It needs to also do:

   from __future__ import absolute_import, division, print_function, unicode_literals

This will give you syntax much closer to Python 3 in Python 2.7. The remaining differences can be papered over with function and class helpers. Well, except for metaclasses, but I doubt you'd have need for those in a single-file script.