Use correct return type annotation for BaseException.with_traceback #4298
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The return type of the BaseException.with_traceback() method [1] is not
specific enough. The return type is guaranteed to be of the same type as
‘self’, which is usually a subclass of BaseException.
In fact, .with_traceback() returns ‘self’:
Fix the annotation to reflect this using the self-type annotation
technique described in PEP484 [2], which is supported by (at least)
mypy [3].
[1] https://docs.python.org/3/library/exceptions.html#BaseException.with_traceback
[2] https://www.python.org/dev/peps/pep-0484/#annotating-instance-and-class-methods
[3] python/mypy#1212