I used modern python (3.4 to be precise) in production more than year ago. It is ready for production for a LONG time.
Most of the libraries and frameworks now work with python 3, and some support only python3 like Django 2.x
So unless you are constrained by a library or framework that does not work on python3 (like web2py, as mentioned by @Remco) , there is no reason to use "legacy" python aka python2