I think that just means "numpy hasn't yet been rebuilt for Python 3.8".
(In Python modules that include a C extension, the .py files are shared
but the C part is compiled separately for each Python version.)
As there are ~500 modules requiring such a rebuild
(https://release.debian.org/transitions/html/python3.8.html), this may
take some time. (numpy itself will probably be a source upload, to move
1.17 from experimental to unstable.)
For testing, it may be easiest to use an Ubuntu chroot for now.