The following code causes the interpreter to crash:
import xml.etree.ElementTree
class X:
    def __del__(self):
        elem.clear()
elem = xml.etree.ElementTree.Element('elem')
elem.text = X()
elem.clear()

This is because _elementtree_Element_clear_impl() decrefs self->text in an
unsafe manner.
For the same reason, but for self->tail, a crash would happen if we replaced
'elem.text = X()' with 'elem.tail = X()'.


Similarly, the following code also causes the interpreter to crash:
import xml.etree.ElementTree
class X:
    def __del__(self):
        elem.clear()
elem = xml.etree.ElementTree.Element('elem')
elem.text = X()
elem.text = X()

This is because element_text_setter() decrefs self->text in an unsafe manner.
element_tail_setter() does the same for self->tail, so again, if we replaced
'elem.text = X()' with 'elem.tail = X()', we would also get a crash.

As serhiy pointed out in a comment in PR 3924, setting self->text or self->tail to
NULL might lead to an assertion failure, so we should also prevent the following
assertion failure (and the similar one for tail):
import xml.etree.ElementTree
class X:
    def __del__(self):
        elem.text
        
elem = xml.etree.ElementTree.Element('elem')
elem.text = X()
elem.__setstate__({'tag': None}) # implicitly also set elem.text to None

New changeset 39ecb9c71b6e55d8a61a710d0144231bd88f9ada by Serhiy Storchaka (Oren Milman) in branch 'master':
bpo-31728: Prevent crashes in _elementtree due to unsafe cleanup of Element.text and Element.tail (#3924)
https://github.com/python/cpython/commit/39ecb9c71b6e55d8a61a710d0144231bd88f9ada

New changeset a8ac71d15f2a4aef83a8954a678cc12545ce517c by Serhiy Storchaka (Miss Islington (bot)) in branch '3.6':
[3.6] bpo-31728: Prevent crashes in _elementtree due to unsafe cleanup of Element.text and Element.tail (GH-3924) (#3945)
https://github.com/python/cpython/commit/a8ac71d15f2a4aef83a8954a678cc12545ce517c

New changeset f15058a697de12b0efe6c7ebc38fe61a993bb5d5 by Serhiy Storchaka (Oren Milman) in branch '2.7':
[2.7] bpo-31728: Prevent crashes in _elementtree due to unsafe cleanup of Element.text and Element.tail (GH-3924) (#3950)
https://github.com/python/cpython/commit/f15058a697de12b0efe6c7ebc38fe61a993bb5d5

Thank you Oren!