USES=python3.10+ and RUN_DEPENDS

hi all,

i'm trying to port an application that requires Python 3.10+ or later,
while the default Python is still 3.9.

this is my Makefile: https://git.sr.ht/~lw/freebsd-ports/tree/sublime-music/item/audio/py-sublime-music/Makefile

running poudriere testport initially produces several messages like
this, one for each port in RUN_DEPENDS:

[00:02:48] Ignoring devel/py-dateutil@py310 | py310-dateutil-2.8.2: Unknown flavor 'py310', possible flavors: py39
[00:02:48] Ignoring databases/py-peewee@py310 | py310-peewee-3.15.0: Unknown flavor 'py310', possible flavors: py39
[00:02:48] Ignoring devel/py-dataclasses-json@py310 | py310-dataclasses-json-0.5.9: Unknown flavor 'py310', possible flavors: py39

it then installs python310 to build the port, but later fails like this:

=======================<phase: run-depends    >============================
===== env: DEVELOPER_MODE=yes USE_PACKAGE_DEPENDS_ONLY=1 USER=root UID=0 GID=0
===>   py310-sublime-music-0.12.0 depends on package: py310-gobject3>0 - not found
===>   py310-sublime-music-0.12.0 depends on package: /packages/All/py310-gobject3-3.42.2.pkg - not found
===>   USE_PACKAGE_DEPENDS_ONLY set - not building missing dependency from source
*** Error code 1

could someone please explain what i'm doing wrong here?

	thanks, lw.