[issue43988] Add test.support.assert_dissallow_instantiation

[Erlend Egeberg Aasland]

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


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keywords: +patch
pull_requests: +24447
stage:  -> patch review
pull_request: https://github.com/python/cpython/pull/25757

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