python@3.9: use `altinstall` target
python@3.9: use `altinstall` target

This makes sure that Python does not install major-versioned files (e.g.
`python3`, `python3-config`) and instead only installs files with both a
major and minor version (e.g. `python3.9`, `python3.9-config`).

This will allow us to have multiple versions of Python linked into
`HOMEBREW_PREFIX` without conflicts.

If users want to call `python3.9` as `python3`, then they can add
`libexec/"bin"` to their `PATH`.

Going forward, this will help simplify migrating to newer versions of
Python, since we will not need to make the newer version keg-only while
dependent formulae are migrated.

See discussion at Homebrew/discussions#2776, and documentation at

    https://github.com/python/cpython#installing-multiple-versions

Closes #87075.

filcab/homebrew-core,lemzwerg/homebrew-core,sjackman/homebrew-core,lemzwerg/homebrew-core,makigumo/homebrew-core,filcab/homebrew-core,sjackman/homebrew-core,nandub/homebrew-core,nbari/homebrew-core,adam-moss/homebrew-core,chrisfinazzo/homebrew-core,nandub/homebrew-core,stgraber/homebrew-core,chrisfinazzo/homebrew-core,Homebrew/homebrew-core,stgraber/homebrew-core,adam-moss/homebrew-core,nbari/homebrew-core,makigumo/homebrew-core,Homebrew/homebrew-core