[issue45254] HAS_SHMEM detection logic is duplicated in implementation and tests

[Terry J. Reedy]

Change by Terry J. Reedy <tjreedy at udel.edu>:


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nosy: +davin, pitrou
versions:  -Python 3.8

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue45254>
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