How do I backport a package that depends on another package which only exists in testing?

I'm trying to backport mailman3-full from Debian Bullseye to Debian Buster. I've followed the official guide , but when executing sudo mk-build-deps --install --remove I get an error:

Removing mailman3-build-deps:amd64 because I can't find python3-importlib-resources:amd64

I've researched that package, and apparently it's only found on Bullseye (testing), so I can't just install it. How should I proceed to backport mailman3-full? Maybe I also have to backport python3-importlib-resources? I've also tried adding testing repositories and just installing mailman3-full from there, but then it wants to upgrade a lot of packages to the testing version, which I'm not comfortable with.