[issue43988] Add test.support.assert_dissallow_instantiation

[Erlend E. Aasland]

Change by Erlend E. Aasland <erlend.aasland at innova.no>:


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pull_requests: +24985
pull_request: https://github.com/python/cpython/pull/26392

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<https://bugs.python.org/issue43988>
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