[issue40108] Improve error message for -m option when .py is present

[Pablo Galindo Salgado]

Change by Pablo Galindo Salgado <pablogsal at gmail.com>:


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keywords: +patch
nosy: +pablogsal
nosy_count: 2.0 -> 3.0
pull_requests: +18596
stage:  -> patch review
pull_request: https://github.com/python/cpython/pull/19239

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue40108>
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