conformance: allow emitting an error on missing return with Any annotation

Pycroscope produces an error on the existing open() function in this file
because it is annotated as returning Any, but doesn't have a return statement.
There's some existing discussion of this case in python/mypy#10297
showing that it is intentional for mypy to not error here, but I don't think
we need to prohibit type checkers from emitting an error in this case
if they choose.

Author: JelleZijlstra

conformance/tests/literals_interactions.py

@@ -43,7 +43,7 @@ def open(path: _PathType, mode: str) -> IO[Any]:
 
 
 def open(path: _PathType, mode: Any) -> Any:
-    pass
+    raise NotImplementedError
 
 
 assert_type(open("path", "r"), IO[str])