Add exhaustive check for mypy

Summary:
Ran into this when refactoring `PartitionSetDefinition`.

This idiom appears in https://hakibenita.com/python-mypy-exhaustive-checking and
python/mypy#5818 - this is a nice to ensure exhaustion
when doing conditionals on enums.

Test Plan: N/A

Reviewers: dgibson, cdecarolis, alangenfeld

Reviewed By: dgibson

Differential Revision: https://dagster.phacility.com/D7642

python_modules/dagster/dagster/check/__init__.py

@@ -859,3 +859,7 @@ def class_param(obj: Any, param_name: str) -> Union[ParameterCheckError, type]:
             f'Param "{param_name}" is not a class. Got {repr(obj)} which is type {type(obj)}.'
         )
     return obj
+
+
+def assert_never(value: NoReturn) -> NoReturn:
+    failed(f"Unhandled value: {value} ({type(value).__name__})")