Separate the installer `Shell profile updater` postinstall script from the `Update Shell Profile.command` to enable more robust error handling.
(cherry picked from commit 781eb1a688e0575a38c79575493f3ff83208a33e)

Co-authored-by: Ned Deily <nad@python.org>

Co-authored-by: Erlend E. Aasland <erlend@python.org>

* Remove the Tools/scripts/2to3 script.
* Remove the Lib/test/test_lib2to3/ directory.
* Doc/tools/extensions/pyspecific.py: remove the "2to3fixer" object
  type.
* Makefile and PC/layout/main.py no longer compile lib2to3 grammar
  files.
* Update Makefile for 2to3 removal.

Remove the bundled setuptools wheel from ensurepip, and stop installing setuptools in environments created by venv.

Co-Authored-by: Hugo van Kemenade <hugovk@users.noreply.github.com>
Co-authored-by: C.A.M. Gerlach <CAM.Gerlach@Gerlach.CAM>
Co-authored-by: Oleg Iarygin <oleg@arhadthedev.net>

* Move Lib/lib2to3/tests/ to Lib/test/test_lib2to3/.
* Remove Lib/test/test_lib2to3.py.
* Update imports.
* all_project_files(): use different paths and sort files
  to make the tests more reproducible.
* Update references to tests.

Adds a simple check for whether or not the package is being installed in the GUI or using installer on the command line. This addresses an issue where CLI-based software management tools (such as Munki) unexpectedly open Finder windows into a GUI session during installation runs.

Currently, "pip3 install --upgrade pip" unconditionally installs a
"pip" alias even for Python 3.  If a user has an existing Python 3.x
installed from a python.org macOS installer and then subsequently
manually updates to a new version of pip, there may now be a stray
"pip" alias in the Python 3.x framework bin directory which can cause
confusion if the user has both a Python 2.7 and 3.x installed;
if the Python 3.x fw bin directory appears early on $PATH, "pip"
might invoke the pip3 for the Python 3.x rather than the pip for
Python 2.7.  To try to mitigate this, the macOS installer script
for the ensurepip option will unconditionally remove "pip" from
the 3.x framework bin directory being updated / installed.  (The
ambiguity can be avoided by using "pythonx.y -m pip".)