[issue42557] Make asyncio.__main__ reusable, also adding a preamble feature.

[Berry Schoenmakers]

Change by Berry Schoenmakers <l.a.m.schoenmakers at tue.nl>:


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keywords: +patch
pull_requests: +22889
stage:  -> patch review
pull_request: https://github.com/python/cpython/pull/24055

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue42557>
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