Source: scipy Followup-For: Bug #1056888 Using cython3-legacy sounds like a reasonable workaround.
I presume the latest version of scipy has been updated to use cython 3, but we won't know until https://salsa.debian.org/salsa/support/-/issues/360 is dealt with. cf. https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1042459
