gh-153511: Removes Tools/msi, Tools/nuget, and legacy py.exe launcher sources

[zooba]

• Issue: Remove deprecated Windows installer #153511

[zooba]

For anyone actually reviewing this, the PC/layout/README.md file is an addition, not a removal, but it's hard to find in the diff view. Initial contents are here but that link won't update with new commits.

[hugovk]

A bunch of suggestions from Claude:

---

Orphaned build files (clear removals)

• PCbuild/pyshellext.vcxproj and PCbuild/pyshellext.vcxproj.filters — the sources
  (PC/pyshellext.cpp/.def/.rc) were deleted and the project was already dropped from
  pcbuild.sln/pcbuild.proj, so these two files are dead.
• PCbuild/venvlauncher.vcxproj.filters:17 and PCbuild/venvwlauncher.vcxproj.filters:22
  still reference the deleted ..\PC\pylauncher.rc; the .vcxproj files were updated to
  python_exe.rc, but the .filters weren't.

References to deleted files

• README.rst:79-80 — links to the deleted Tools/msi/README.txt ("To build Windows
  installer, see …").
• PC/store_info.txt:30,46 — instructions call the deleted Tools/msi/make_appx.ps1.
• .github/CODEOWNERS:192-193,196 — entries for Tools/msi/, Tools/nuget/, and
  PC/launcher.c.
• .gitignore:53,161 — Tools/msi/obj and the "not Tools/msi/core" comment.
• .gitattributes:22 — PC/classicAppCompat.* binary (the files are gone; appxmanifest.py
  deliberately keeps runtime logic for older releases, but the attribute is dead).
• PCbuild/readme.txt:143-160 — still documents the pylauncher, pywlauncher, and
  pyshellext sub-projects.

Dead CI/tooling logic

• Tools/build/compute-changes.py — the whole run_windows_msi path (lines 120, 163-164,
  226, 248-250, 291-293, 330, 345) triggers on the deleted reusable-windows-msi.yml and
  Tools/msi/; build.yml/reusable-context.yml no longer consume it. The matching test data
  in Lib/test/test_tools/test_compute_changes.py:114-115 too.
• .azure-pipelines/windows-layout-steps.yml:15 — -i test_launcher excludes a test that no
  longer exists.

Stale doc/comment pointers to PC/launcher.c

• Include/internal/pycore_magic_number.h:314 — "PC/launcher.c must also be updated"
  (magic-number bump instructions).
• InternalDocs/interpreter.md:312 — same note, with a link to the deleted file. These
  presumably should now point at the launcher's new home (pymanager) or be dropped.

[zooba]

Thanks, Claude (I wish we hadn't turned off Copilot reviews here, at least those would come with suggestions that I could batch and commit from the UI...)

[hugovk]

Also a couple of docs sections that could be removed; or possibly just keep the red "Deprecated since version 3.14" paragraph, change the tense, and update deprecated:: 3.14 to deprecated-removed:: 3.14 3.16:

• 4.11. The full installer (deprecated)
• 4.12. Python launcher for Windows (deprecated)