[issue20490] Show clear error message on circular import

[Pablo Galindo Salgado]

Pablo Galindo Salgado <pablogsal at gmail.com> added the comment:

This PR introduced a reference leak tracked in:

https://bugs.python.org/issue38090

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nosy: +pablogsal

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue20490>
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