Hi Fred,
this is a fix introduced new in 3.1 to make sure that the wrong interpreter isn't used silently when tox is misconfigured. I wasn't involved in the fix and lack the details, but your config looks like there should be no such warning. When you say that py36 and py37 weren't found at all: do you mean that the warning shadows an actual error? If this is the case, it would be great if you could open an issue with a quick explanation how to reproduce this.
Here are the details for this issue and the changes, if you want to look further into this:
Cheers,
Oliver