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`.
A user may wish to use two use two brew-installed Python packages
together. For example, one might want to `import numpy` when using
`jupyterlab` or `ptpython`.
Currently, the only ways to do this I'm aware of is with some hacking of
`PYTHONPATH` or the creation of `.pth` files in a formula's prefix.
A better solution is to allow the virtualenvs that `brew` creates to
have access to system site-packages by default, so that `import numpy`
inside `ptpython` or `jupyterlab` just works.
Partially resolvesHomebrew/homebrew-core#76950.
