Incompatible return type error doesn't quote types #3771

The error message for an incompatible return type doesn't have quotation marks around the types. This makes it difficult to programmatically process the output of mypy to filter "expected" errors.

Example output:
Error: Incompatible return value type (got List[Type1], expected List[Type2])

Expected output:
Error: Incompatible return value type (got "List[Type1]", expected "List[Type2]")