On Saturday, 25 April 2020 17:02:05 BST Michael Orlitzky wrote:
> On 4/25/20 11:55 AM, tuxic@posteo.de wrote:
> > ##  ... PYTHON_SINGLE_TARGET="python3_6 python3_8 -python3_7"
> > ##
> > ##    The following REQUIRED_USE flag constraints are unsatisfied:
> > ##      exactly-one-of ( python_single_target_python3_6
> > python_single_target_python3_7 python_single_target_python3_8 )
> "Single target" means you've gotta pick just one.

eselect python update/cleanup options should help you move along with this 
package without having to set up a specific single python target for it.  Let 
it inherit the system version.