Backport the test-only parts of the Tcl/Tk 8.7/9.0/9.1 test adaptations so that test_tkinter and test_ttk pass when Python 3.13 is built against Tcl/Tk 9.0 or 9.1 (for example the Homebrew build used on macOS — see gh-128846), while still passing against the Tcl/Tk 8.6 that 3.13 ships.

These adaptations were made during 3.14/3.15 development and never backported to 3.13, so building 3.13 against Tk 9.0 currently produces ~90 spurious test_tkinter failures and several test_ttk failures (error-message wording such as doesn't→does not and can't→cannot find busy window, pixel rounding/scaling, clipped negative sizes, identify() returning padding instead of focus, etc.).

This combines the test changes from:

• Update tkinter to use Tk 8.6.15 or (better) Tk 9.0 #124111: Update tkinter for compatibility with Tcl/Tk 9.0.0 (gh-124111: Update tkinter for compatibility with Tcl/Tk 9.0.0 #124156)
• Update tkinter to use Tk 8.6.15 or (better) Tk 9.0 #124111: test macOS CI with Tk 9 (gh-124111: test macOS CI with Tk 9 #137424)
• Tkinter tests fail with Tk 8.7, 9.0 and 9.1 #145736: Fix Tkinter tests for Tk 8.7, 9.0 and 9.1 (gh-145736: Fix Tkinter tests for Tk 8.7, 9.0 and 9.1 #145738)

The corresponding non-test changes (Modules/_tkinter.c, Lib/tkinter/ttk.py, PCbuild and CI configuration) are not backported. The \z regular-expression escape introduced by gh-133306 is not available in the 3.13 re module, so \Z is kept.

The NotebookTest.test_traversal hardening is handled separately by GH-153415 (already merged) and is not part of this PR.

Verified locally against Tcl/Tk 8.6, 9.0.4 and 9.1b1 (the affected test_tkinter and test_ttk methods pass on all three); the set of collected test cases is unchanged, so no test coverage is dropped.