Python 3.5 is EOL already, and Blender never used version 3.6 (it went
straight to 3.7), hence 3.6 was never tested explicitly.
Python 3.7 or newer is required from now on.
This reverts commit 46466de7db924d374a97c74219db9694b9a67bbd.
Tox wasn't set up correctly for this bump, and we may want to keep
compatibility with older Blenders as well.