When Homebrew/homebrew-core#107517 is merged, builds will no longer be
able to find `python@3.9` as `python3`. This is also what is likely to
happen to `python@3.10` when we add a `python@3.11`.
This is likely to break many builds, so let's make sure they can keep
finding a `python3` for formulae that don't have a dependency on the
latest Python3.
This is arguably something we should've done earlier: it also means that
builds that go looking for an unversioned `python` end up finding our
Python3 (whenever present in the build environment) instead of, say,
`/usr/bin/python` which is typically Python2.
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