Some formulae are flexible about the version of Python3 that they use.
However, when we use `#detected_python_shebang` on these formulae, they
become coupled to the specific version of Python3 declared in the
formula.
This is harmful because
1. it prevents us from using `uses_from_macos "python"` even in formulae
where we should be able to
2. it forces us to rebuild the formula whenever we make changes to the
Python dependency when nothing but the shebang would have changed as
a consequence of the rebuild
For an example of this, see Homebrew/homebrew-core#107905.
I'd also like to do this to get rid of some really terrible hacks we
have in `glib-utils` as a means of decoupling `glib` from the specific
versioned Python dependency it used to have.
See Homebrew/homebrew-core#103916, or Homebrew/homebrew-core#106045 for
a proposal to give the same treatment to `gobject-introspection`.
