Python 3.7 to become the default target
On 2020-05-06 (or later), Python 3.7 will replace Python 3.6 as one of the default Python targets for Gentoo systems. The new default values will be:
PYTHON_TARGETS="python2_7 python3_7" PYTHON_SINGLE_TARGET="python3_7"
If you have not overriden these variables on your system, then your package manager will switch to the new targets immediately. In order to avoid dependency conflicts, please clean stray packages up and rebuild/upgrade all packages with USE flag changes after the change, e.g.:
emerge --depclean emerge -1vUD @world emerge --depclean
Please note that during the system upgrade some of the package dependencies may temporarily become missing. While this should not affect programs that are already fully loaded, it may cause ImportErrors when starting Python scripts or loading additional modules (only scripts running Python 3.6 are affected).
In order to improve stability of the upgrade, you may choose to temporarily enable both targets, i.e. set in /etc/portage/package.use or its equivalent:
*/* PYTHON_TARGETS: python3_6 python3_7 */* PYTHON_SINGLE_TARGET: -* python3_6
This will cause the dependencies to include both Python 3.6 and 3.7 support whenever possible, throughout the next system upgrade. Once all packages are updated, you can restart your scripts, remove the setting and start another upgrade in order to cleanly remove Python 3.6.
There are still some Gentoo packages that do not support Python 3.7. We will be pushing updates to these packages as time permits. However, some of them will probably be removed instead.
If you would like to postpone the switch to Python 3.7, you can copy the current value of PYTHON_TARGETS and/or PYTHON_SINGLE_TARGET to /etc/portage/package.use or its equivalent:
*/* PYTHON_TARGETS: -python3_7 python3_6 */* PYTHON_SINGLE_TARGET: -* python3_6
If you would like to migrate your systems earlier, you can do the opposite. Note that if you are running ~arch, you may want to go straight for Python 3.8 at this point.
Please note that this switch is long overdue. Python 3.6 is no longer receiving bug fixes. Its planned end-of-life is 2021-12-23 but we will probably remove it earlier than that. 
All above snippets assume using package.use to control USE flags. If you still have make.conf entries for these targets, you need to remove them. While using make.conf is possible, it is strongly discouraged as it does not respect package defaults. The latter can help you keep some of Python 3.6 packages without the need to explicitly toggle flags per-package.