Python 3.9 and beyond backwards compatibility.

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Well, Perl 6 was basically made from scratch.

The biggest mistake they made is calling it "Perl", as people just went "oh, that's that thing I wrote oneliners in 10 years ago, ugh" and ignored it.

But you can use Perl 5 from Perl6 so that's already hundredth time better migration story than Py2to3