Earlier quoted context omitted.
Compiling Python to Javascript is always going to be incomplete. (For example, it doesn't make much sense to open() a file from the browser!) However, we run full CPython (2.7 and 3.6) on the server, so any language feature you like is just a function call away :)
Ok, so I can run 3.6 code on the server and return the output? As Py2 is EOL in about two years it feels risky for anything to be built on it.
The client-side code is Python 2 only for now, but we're already upgrading our Python-to-Javascript compiler to include Python 3 syntax.