Lazy load SciPy when moving minimum req to 1.9

[prisae]

Just something to keep in mind: From SciPy>=1.9 onwards, we can do, e.g.,

import scipy as sp

def myfunc():
    sp.interpolate.xyz
    sp.integrate.xyz
    ...

The submodules, e.g., interpolate, are lazy loaded within SciPy>=1.9. Before that, one had to do

from scipy import interpolate
from scipy import integrate
...

Using this throughout would save roughly 50% of the import time of discretize (crucial for the responsiveness of CLI tools that use discretize).