gh-135640: Adds type checking to ElementTree.ElementTree constructor, plus relevant tests

[abstractedfox]

Addresses issue #135640 by adding type checking to the constructor and to ElementTree._setroot, since both have the opportunity to assign _root. In addition to addressing the primary issue (possible data loss), this makes relevant tracebacks more descriptive.

It looks like there had once been asserts that did these checks. I couldn't find rationale for why they were commented out in blame (it looks like this was done on svn, and I'm not sure where to look to find those commit messages). I opted to use raise instead as I felt it would be more descriptive.

At present, the way iselement is written still allows an object of the wrong type to pass through if it has a tag attribute. Commit a72a98f shows there were once comments indicating that the current implementation was done for a reason, but it isn't clear to me why that is. I left it alone for now, but changing it to this:

def iselement(element):
    return isinstance(element, Element)

...also passed tests. If there's no disagreement, I think it would be good to change that too.

• Issue: Calling ElementTree.write on an ElementTree instance where ElementTree._root is the wrong type causes possible data loss #135640

[python-cla-bot]

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