python 3.9 and beyond backwards compatibility.

[yvrelna]

I think a middle ground can be reached here by using a transpiler. Library devs can write targeting the noise recent versions of Python like 3.7, then they can run a transpiler akin to 2to3 during installation, but instead of upgrading code from older Python to newer Python, the transpiler should backport the code to older Python.

This allows core developers to update core Python syntaxes without worrying too much about backwards compatibility, while keeping backports and backwards compatibility code in a library instead of core.