[issue42997] Improve error message for missing : before suites

[Lysandros Nikolaou]

Lysandros Nikolaou <lisandrosnik at gmail.com> added the comment:

I propose to go with the first approach. I really like that and I can see us using it in various place in the future. It's basically a positive lookahead with a cut, but the extra feature that it raises a SyntaxError, right?

I'll have a look at the PR now.

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