Tighten return types of always-succeeding to_affine()

[Tomaz-Vieira]

Some transformations always succeed when calling .to_affine(), but their method signature still returns Affine | None. This patch tightens their types to simply return Affine when we know they always succeed. This is still compatible with the parent signature (i.e. Affine is a subtype of Affine | None), but is convenient for code that is dealing with concrete children of Transform, since they don't have to assert that to_affine() is not None:

# before:
def use_scale(scale: Scale):
    scale_affine = scale.to_affine()
    assert scale_affine is not None
    use_affine(scale_affine)

# after
def use_scale2(scale: Scale):
    scale_affine = scale.to_affine()
    use_affine(scale_affine)

[clbarnes]

If type checkers are happy with this narrowing, that's perfect!