language/python: create venvs with access to system site packages

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 resolves Homebrew/homebrew-core#76950.

Library/Homebrew/language/python.rb

@@ -218,7 +218,7 @@ module Language
         def create
           return if (@venv_root/"bin/python").exist?
 
-          @formula.system @python, "-m", "venv", @venv_root
+          @formula.system @python, "-m", "venv", "--system-site-packages", @venv_root
 
           # Robustify symlinks to survive python patch upgrades
           @venv_root.find do |f|

Library/Homebrew/test/language/python/virtualenv_spec.rb

@@ -15,7 +15,7 @@ describe Language::Python::Virtualenv::Virtualenv, :needs_python do
 
   describe "#create" do
     it "creates a venv" do
-      expect(formula).to receive(:system).with("python", "-m", "venv", dir)
+      expect(formula).to receive(:system).with("python", "-m", "venv", "--system-site-packages", dir)
       virtualenv.create
     end
   end