gh-97527: IDLE: protect macosx Tk() call when no GUI (#97530)

Only call tkinter.tk and its follow-up code in _init_tk_type when requires('gui')
does not raise.  This function can be called as an unintended side-effect of
calling other idlelib code as part of tests on macOS without a GUI enabled.

Lib/idlelib/macosx.py

@@ -4,6 +4,7 @@ A number of functions that enhance IDLE on macOS.
 from os.path import expanduser
 import plistlib
 from sys import platform  # Used in _init_tk_type, changed by test.
+from test.support import requires, ResourceDenied
 
 import tkinter
 
@@ -14,12 +15,15 @@ import tkinter
 _tk_type = None
 
 def _init_tk_type():
-    """
-    Initializes OS X Tk variant values for
-    isAquaTk(), isCarbonTk(), isCocoaTk(), and isXQuartz().
+    """ Initialize _tk_type for isXyzTk functions.
     """
     global _tk_type
     if platform == 'darwin':
+        try:
+            requires('gui')
+        except ResourceDenied:  # Possible when testing.
+            _tk_type = "cocoa"  # Newest and most common.
+        else:
             root = tkinter.Tk()
             ws = root.tk.call('tk', 'windowingsystem')
             if 'x11' in ws: