The Save & Run button defaults to Python 3.7. If you want to use Python 2, you can do so by inserting a special line at the top of your file, called a hashbang:
#!/usr/bin/python2.7 print "hello from python 2"
Will load Python 2,
Or, for example:
#!/usr/bin/python3.4 print("hello from python 3.3")
will load python 3.4 instead of 3.6.
You can also use a hashbang to run your script in a virtualenv that you've defined by pointing it at the python executable in the virtualenv.
#!/home/myusername/.virtualenvs/myvenv/bin/python print("hello from python in a virtualenv")
NB the hashbang has to be the very first line in the file.