[3.10] bpo-43988: Add test.support.check_disallow_instantiation() (GH-25757)

[erlend-aasland]

(cherry picked from commit 4f72526, fbff538, and 8cec740)

Co-authored-by: Erlend Egeberg Aasland erlend.aasland@innova.no

https://bugs.python.org/issue43988

Automerge-Triggered-By: GH:vstinner

[erlend-aasland]

I'll backport GH-26392, GH-26394, and GH-26412 after this has been merged.

EDIT: the two former PR's have been included in this backport PR, as they belong to the same bpo issue.

[vstinner]

LGTM. But please fix the doc.

[vstinner]

LGTM, thanks.

[miss-islington]

@erlend-aasland: Status check is done, and it's a failure ❌ .

[erlend-aasland]

test_ttk_guionly failed on Ubuntu in Azure Pipelines.

2021-06-23T22:15:57.5133290Z ======================================================================
2021-06-23T22:15:57.5133791Z FAIL: test_configure_to (tkinter.test.test_ttk.test_widgets.SpinboxTest)
2021-06-23T22:15:57.5134904Z ----------------------------------------------------------------------
2021-06-23T22:15:57.5135381Z Traceback (most recent call last):
2021-06-23T22:15:57.5135890Z   File "/home/vsts/work/1/s/Lib/tkinter/test/test_ttk/test_widgets.py", line 1175, in test_configure_to
2021-06-23T22:15:57.5136405Z     self._click_increment_arrow()  # 5
2021-06-23T22:15:57.5136909Z   File "/home/vsts/work/1/s/Lib/tkinter/test/test_ttk/test_widgets.py", line 1135, in _click_increment_arrow
2021-06-23T22:15:57.5137627Z     self.assertRegex(self.spin.identify(x, y), r'.*uparrow\Z')
2021-06-23T22:15:57.5138321Z AssertionError: Regex didn't match: '.*uparrow\\Z' not found in ''

[miss-islington]

@erlend-aasland: Status check is done, and it's a failure ❌ .

[miss-islington]

@erlend-aasland: Status check is done, and it's a failure ❌ .

[miss-islington]

@erlend-aasland: Status check is done, and it's a success ✅ .