Compile classy with Cython version 3.0+

[EBHolm]

Since Cython 3.0 (which is the new standard version), it is illegal to cpdef variables; instead, one should use cdef. This has resulted in classy being unable to install when the user has the newest version of cython installed (see lesgourg/class_public#531). An easy fix for now is to simply replace cpdef with cdef the 4-5 places it occurs in classy.pyx. class_public has not fixed this yet, but it would be an advantage to do it here on the fork itself to avoid users being unable to install.

Later on, more things will be deprecated, but at that point I think it's best to follow whatever solution will be implemented on class_public.