As of 3.9.1rc1 (and coming soon to 3.10.x and 3.8.x), we now provide a new additional macOS universal2 installer variant that, for various reasons, provides a built-in version of Tcl/Tk 8.6.10, instead of 8.6.8 as has been provided with the 10.9 installer variant.  Using 8.6.10 on nacOS, six test_ttk_guionly tests fail while passing using 8.6.8 ib nacOS. Similar results were obtained when running the tests on either macOS 10.15.7 or 11.0.1. The failures:

======================================================================
FAIL: test_identify (tkinter.test.test_ttk.test_widgets.ComboboxTest)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/Library/Frameworks/Python.framework/Versions/3.9/lib/python3.9/tkinter/test/test_ttk/test_widgets.py", line 334, in test_identify
    self.assertIn(self.entry.identify(5, 5),
AssertionError: '' not found in ('textarea', 'Combobox.button')

======================================================================
FAIL: test_identify (tkinter.test.test_ttk.test_widgets.EntryTest)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/Library/Frameworks/Python.framework/Versions/3.9/lib/python3.9/tkinter/test/test_ttk/test_widgets.py", line 334, in test_identify
    self.assertIn(self.entry.identify(5, 5),
AssertionError: '' not found in ('textarea', 'Combobox.button')

======================================================================
FAIL: test_command (tkinter.test.test_ttk.test_widgets.SpinboxTest)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/Library/Frameworks/Python.framework/Versions/3.9/lib/python3.9/tkinter/test/test_ttk/test_widgets.py", line 1155, in test_command
    self.assertTrue(success)
AssertionError: [] is not true

======================================================================
FAIL: test_identify (tkinter.test.test_ttk.test_widgets.SpinboxTest)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/Library/Frameworks/Python.framework/Versions/3.9/lib/python3.9/tkinter/test/test_ttk/test_widgets.py", line 334, in test_identify
    self.assertIn(self.entry.identify(5, 5),
AssertionError: 'Spinbox.field' not found in ('textarea', 'Combobox.button')

======================================================================
FAIL: test_bbox (tkinter.test.test_ttk.test_widgets.TreeviewTest)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/Library/Frameworks/Python.framework/Versions/3.9/lib/python3.9/tkinter/test/test_ttk/test_widgets.py", line 1353, in test_bbox
    self.assertIsBoundingBox(bbox)
  File "/Library/Frameworks/Python.framework/Versions/3.9/lib/python3.9/tkinter/test/widget_tests.py", line 197, in assertIsBoundingBox
    self.assertIsInstance(bbox, tuple)
AssertionError: '' is not an instance of <class 'tuple'>

======================================================================
FAIL: test_identify (tkinter.test.test_ttk.test_widgets.WidgetTest)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/Library/Frameworks/Python.framework/Versions/3.9/lib/python3.9/tkinter/test/test_ttk/test_widgets.py", line 68, in test_identify
    self.assertEqual(self.widget.identify(
AssertionError: '' != 'label'
+ label


https://www.python.org/downloads/release/python-391rc1/