In case you missed the news, Fedora 28 is now available as a template in Qubes OS 4.0. Fedora 26 will end of life on 2018-06-01, means this is a good time for everyone to upgrade. Use the following command in your dom0 to install the template. The template is more than 1GB in size, means it will take some time to download.

$ sudo qubes-dom0-update qubes-template-fedora-28

After installation, remember to start the template, and update, and also install all the required applications there. Next step would be to use this template everywhere.

Btw, we do have the latest Python 3.6.5 in Fedora 28 :)

$ python3
Python 3.6.5 (default, Mar 29 2018, 18:20:46)
[GCC 8.0.1 20180317 (Red Hat 8.0.1-0.19)] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>>