gh-36590: Allow to import `sage.cpython` module multiple times
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I don't really know why but pytest is sometimes importing the same
module multiple times (I think once for the test collection and once for
the execution). Importing cpython twice however fails since the second
time `_ExtensionFileLoader.get_source` is already deleted.
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URL: https://github.com/sagemath/sage/pull/36590
Reported by: Tobias Diez
Reviewer(s): Matthias Köppe
