Well, I tried, but didn't make it.
I added
=dev-lang/python-3.9.13
to /etc/portage/package.unmask, so that it could be installed, and I added
PYTHON_TARGETS="python2_7 python3_9"
to /etc/portage/make.conf. (I also tried adding a line PYTHON_SINGLE_TARGET="python3_9" to make.conf, but that seemed to cause more problems, so I got rid of that.)
I also had several packages in /etc/portage/package.use with flags like "PYTHON_TARGETS: -* python3_7", so I changed all of those to python3_9.
After that, trying to do an "emerge -auDN @world" gave me pages of errors, so I gave up.
python_merge_err