Note that this goes to package.deprecated, rather than package.mask. 
These are virtuals, so they don't really impact users, and there's
really no point to make Portage complain about them being installed.

# Michał Górny <mgorny@gentoo.org> (2020-03-16)
# Virtuals for Python 2 backports.  Mostly obsolete, and some of them
# are really confusing.  Use $(python_gen_cond_dep ...) instead
# if your package still needs to support Python 2 (or remove it
# for Python 3 only packages).
virtual/python-backports_abc
virtual/python-enum34
virtual/python-funcsigs
virtual/python-futures
virtual/python-ipaddress
virtual/python-pathlib
virtual/python-singledispatch
virtual/python-typing
virtual/python-unittest-mock

-- 
Best regards,
Michał Górny