gh-100933: Improve check_element helper in test_xml_etree

[sobolevn]

Items that this test checks are always str and dict:

• attrib:

  cpython/Modules/_elementtree.c

  Lines 536 to 548 in a9f0144

  	element_get_attrib(ElementObject* self)
  	{
  	/* return borrowed reference to attrib dictionary */
  	/* note: this function assumes that the extra section exists */
  	PyObject* res = self->extra->attrib;
  	if (!res) {
  	/* create missing dictionary */
  	res = self->extra->attrib = PyDict_New();
  	}
  	return res;

• text:

  cpython/Modules/_elementtree.c

  Lines 561 to 564 in a9f0144

  	PyObject *tmp = list_join(res);
  	if (!tmp)
  	return NULL;
  	self->text = tmp;

• tail:

  cpython/Modules/_elementtree.c

  Lines 582 to 585 in a9f0144

  	PyObject *tmp = list_join(res);
  	if (!tmp)
  	return NULL;
  	self->tail = tmp;

As you can see, there are no "custom" types that should be checked differently. isinstance is enough.

test_xml_etree_c is also affected by this change.

[sobolevn]

@ambv I've updated the description of this PR as we discussed. Can you please take a look now? 🙂

[miss-islington]

Thanks @sobolevn for the PR, and @ambv for merging it 🌮🎉.. I'm working now to backport this PR to: 3.10, 3.11.
🐍🍒⛏🤖

[bedevere-bot]

GH-101686 is a backport of this pull request to the 3.11 branch.

[bedevere-bot]

GH-101687 is a backport of this pull request to the 3.10 branch.