Conformance tests: change another empty body to raise NotImplementedError (#2197)

ty emits an error on this line but for the wrong reason (due to the fact that it implicitly returns `None`, which contradicts the return annotation), so the test is showing up as passing for us when it actually shouldn't

Author: AlexWaygood

conformance/tests/generics_paramspec_basic.py

@@ -21,7 +21,7 @@
 
 
 def func1(x: P) -> P:  # E
-    ...
+    raise NotImplementedError
 
 
 def func2(x: Concatenate[int, P]) -> int:  # E