README: Correct Python version compatibility

[akx]

The current versions of Radon do not run on Python 2.

docker run -it python:2.7.10 sh -c 'pip install radon && radon'
Successfully installed colorama-0.4.1 funcsigs-1.0.2 mando-0.7.1 radon-6.0.1 six-1.16.0
$
Traceback (most recent call last):
  File "/usr/local/bin/radon", line 11, in <module>
    sys.exit(main())
  File "/usr/local/lib/python2.7/site-packages/radon/__init__.py", line 10, in main
    from radon.cli import program, log_error
  File "/usr/local/lib/python2.7/site-packages/radon/cli/__init__.py", line 23, in <module>
    from radon.cli.harvest import (
  File "/usr/local/lib/python2.7/site-packages/radon/cli/harvest.py", line 6, in <module>
    from builtins import super
ImportError: No module named builtins
$