Future proofs by checking for Python versions >= 3

[stsievert]

The code checked for exactly Python 3. When Python 4 is released, this code will break and revert to the Python 2 behavior, something we don't want.

That is, I changed

py3 = (sys.version_info[0] == 3)

to

not_py2 = (sys.version_info[0] >= 3)

This is a small refactor, and hence a web-interface change. I haven't tested any of this code.