pylint #112728 Warn about non-string objects in __all__ [validation pending]

Python accepts non-string objects in __all__ as long as modules are only imported via 'import X'. If __all__ is inspected due to a from import and non-string objects are found, an exception is raised. Pylint should be able to detect problems like this.

Example:

def foo():
pass

__all__ = [foo]

Expected:
E????: Invalid object in __all__: foo