> There seems to be some python3_6 and even python2_7 in your error
> output, maybe you have set some older python targets somewhere that
> you've forgotten about?
>

Yeah I was thinking the same.

If 'grep -i python /etc/portage/*' is not empty, try making it that way. On
my system with 1323 packages, having these empty and running emerge -avuUD
world just worked. 3_7 went away and everything was 3_8.