Python: Add VALID_TYPES to class Property#49

To fix Pylint E1101:no-member warning in the resulting Python code that comes from this line in the template: https://github.com/JPEWdev/shacl2code/blob/b9246b21c1621d078ea1f65ee11c438e28ca5038/src/shacl2code/lang/templates/python.j2#L44-L45 Without the VALID_TYPES declared in the `Property` class, Pylint will warn: > Instance of 'Property' has no 'VALID_TYPES' member To fix that, an empty `VALID_TYPES = ()` is added. In `check_type()`, the argument `types` will then be checked first if it is empty. If `types` is empty, then the rest of `check_type()` will be skipped. Note that this PR has changed the semantic of an empty VALID_TYPES. Previously, if VALID_TYPES happens to be empty, it means "nothing is valid". With this PR, it means "everything is valid" (or "nothing to validate"). -- Also sort the imports.