[issue42997] Improve error message for missing : before suites

[Pablo Galindo Salgado]

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


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pull_requests: +23115
stage:  -> patch review
pull_request: https://github.com/python/cpython/pull/24293

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue42997>
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