Fix materializer type compatibility check during loading

[schustmi]

Describe changes

The issubclass(...) checks in BaseMaterializer.can_handle_type were only correct when saving artifacts and not when loading them.

• A materializer can save any artifact that is an associated type or a subclass of an associated type.
• A user can specify a superclass of an associated type as step input and the materializer can be used to load this artifact

class Parent:
  value: int

class Child(Parent):
  another_value: int

class ChildMaterializer(BaseMaterializer):
  ASSOCIATED_TYPES = (Child,)
  
  ...

@step
def producer() -> Child:
  return Child(value=1, another_value=2)

@step
def consumer(parent: Parent) -> None:
  pass

@pipeline
def test():
  consumer(producer())

Pre-requisites

Please ensure you have done the following:

• I have read the CONTRIBUTING.md document.
• If my change requires a change to docs, I have updated the documentation accordingly.
• I have added tests to cover my changes.
• I have based my new branch on develop and the open PR is targeting develop. If your branch wasn't based on develop read Contribution guide on rebasing branch to develop.
• If my changes require changes to the dashboard, these changes are communicated/requested.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Other (add details above)