fix sys.exc_info return type

[tharvik]

sys.exc_info can also return a Tuple of None

If no exception is being handled anywhere on the stack, a tuple containing three None values is returned.

[gvanrossum]

I don't think this will work. If you have code like

a, b, c = sys.exc_info()

then it will complain with an error:

__tmp__.py:4: error: 'Union[Tuple[builtins.type, builtins.BaseException, types.TracebackType], Tuple[void, void, void]]' object is not iterable

[gvanrossum]

My suggestion is to use Tuple[Optional[type], Optional[BaseException], Optional[TracebackType]] as the return type.

[tharvik]

then it will complain with an error:

__tmp__.py:4: error: 'Union[Tuple[builtins.type, builtins.BaseException, types.TracebackType], Tuple[void, void, void]]' object is not iterable

then mypy is wrong, Tuple is iterable, thus the Union should also be iterable. Seems to be already reported on mypy.

For now, I'll use your suggestion, and add a TODO.