That's exactly what you can do! If you open up a Server Module, you'll see a drop-down in the top-right. That will allow you to select a Python 3.6 runtime.
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.
Are you looking at Transcrypt https://www.transcrypt.org/ as a replacement for Skulpt at all? Because its here and it already works great for Python 3.6, it just needs more usage to develop workflows and examples around it and tools like Anvil are a great way to get started with any technology.
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.