In order to avoid circular imports, the schemes should only import
from other modules higher in the following hierarchy:

1. scheme, point

2. spec, ambient_space, morphism

3. toric_morphism, glue

4. homset

5. affine_scheme, projective_scheme, toric_variety

6. algebraic_scheme, fano_toric_variety

7. hypersurface

8. divisor

9. divisor_group

10. toric_divisor

If you nevertheless need something from a module lower in the
hierarchy, you should defer the import. That is, put the import at the
class/method level and not in the module scope. For example, scheme.py
needs spec.py for the default base scheme but contains no "import
spec" at the module level.
