fix: return the input value in assert_type()

It was returning None at runtime, which is incorrect.

Author: h4l

src/denokv/_pycompat/typing.py

@@ -177,4 +177,4 @@ def assert_never(value: Never, /) -> Never:
 else:
 
     def assert_type(val: _T, typ: Any, /) -> _T:
-        pass
+        return val