Without having looked deeper into it: it feels wrong.

A) the math python flavor should provide python3-FOO in all dists, even sle.

B) why would it be needed on sle, not not TW? (The python3-FOO symbol of course)

A) Thanks. I noticed a new submission to SLE to change the flavor from explicit python36 to python3 (https://build.suse.de/request/show/247995), so it will provide the expected version. I'll update the buildrequires.

B) Since the dataclasses is implemented in python37 already, and needed on build time. So it makes sense to SLE/Leap at the moment. Factory uses python38 to build, so it does not need the package to provide the dataclasses. On the other hand, to keep everything in the same page, perhaps it is better to have this general statement without the distro specific tag:

BuildRequires: (python3-dataclasses if python3-base < 3.7)