language/python: add --install-lib to setup_install_args

About 25 formula in homebrew/core pass `--install-lib` to `python3` in
addition to `*setup_install_args` in order to ensure that all files are
installed into the formula's prefix rather than into a `site-packages`
directory in Python's keg.

Let's simplify these formulae by also adding the appropriate
`--install-lib` flag to `setup_install_args`.

Library/Homebrew/language/python.rb

@@ -70,7 +70,7 @@ def self.in_sys_path?(python, path)
       quiet_system python, "-c", script
     end
 
-    def self.setup_install_args(prefix)
+    def self.setup_install_args(prefix, python = "python3")
       shim = <<~PYTHON
         import setuptools, tokenize
         __file__ = 'setup.py'
@@ -84,6 +84,7 @@ def self.setup_install_args(prefix)
         install
         --prefix=#{prefix}
         --install-scripts=#{prefix}/bin
+        --install-lib=#{prefix/site_packages(python)}
         --single-version-externally-managed
         --record=installed.txt
       ]