In an update with several slot collisions (see attachment),  I'm
zero-ing in on the simplest, where a package is to be replaced by the
same package, but with different PYTHON_TARGETS (at least, that's how I
interpret it).

Is there a way to force the PYTHON_TARGETS of the dependency?

Slot collision:

dev-python/jinja:0

   (dev-python/jinja-2.11.2-r1:0/0::gentoo, ebuild scheduled for merge)
USE="-doc -examples -test" ABI_X86="(64)" PYTHON_TARGETS="*python3_8
python3_9 (-pypy3) -python3_6 -python3_7*" pulled in by
dev-python/jinja[python_targets_python3_9(-),python_single_target_python3_9(+)]
required by (sys-auth/pambase-20201103:0/0::gentoo, ebuild scheduled for
merge) USE="nullok passwdqc sha512 -caps -debug -elogind -gnome-keyring
-minimal -mktemp -pam_krb5 -pam_ssh -pwhistory -pwquality -securetty
(-selinux) -systemd" ABI_X86="(64)"


     dev-python/jinja (Argument)

   (dev-python/jinja-2.11.2-r1:0/0::gentoo, installed) USE="-doc
-examples -test" ABI_X86="(64)" PYTHON_TARGETS="*python3_7 (-pypy3)
-python3_6 -python3_8 -python3_9*" pulled in by
dev-python/jinja[python_targets_pypy3(-)?,python_targets_python3_6(-)?,python_targets_python3_7(-)?,python_targets_python3_8(-)?,python_targets_python3_9(-)?,-python_single_target_pypy3(-),-python_single_target_python3_6(-),-python_single_target_python3_7(-),-python_single_target_python3_8(-),-python_single_target_python3_9(-)]
required by (dev-python/sphinx-3.2.1:0/0::gentoo, installed) USE="-doc
-latex -test" ABI_X86="(64)" PYTHON_TARGETS="python3_7 (-pypy3)
-python3_6 -python3_8 -python3_9"

If the package was good enough before, it's likely still good enough. 
Where's the problem?  I've (unsuccessfully) made these attempts:

# */* PYTHON_TARGETS: python3_6 python3_7 python3_8 python3_9
#*/* PYTHON_TARGETS: -python3_6 -python3_7 python3_8 python3_9
# just have one set
*/* PYTHON_TARGETS: python3_8

The sphinx ebuild has no targets, but does have this:

PYTHON_COMPAT=( python3_{6..9} pypy3 )

The emerge command was:

sudo emerge --verbose=y -vuUD --verbose-conflicts  
dev-python/setuptools dev-python/setuptools_scm dev-python/certifi
dev-python/markupsafe dev-python/jinja dev-libs/libxml2