[3.9] bpo-42630: Improve error reporting in Tkinter for absent default root (GH-23781) (GH-23853)

* Tkinter functions and constructors which need a default root window
  raise now RuntimeError with descriptive message instead of obscure
  AttributeError or NameError if it is not created yet or cannot
  be created automatically.

* Add tests for all functions which use default root window.

* Fix import in the pynche script.

(cherry picked from commit 3d569fd6dc)

Lib/idlelib/pyshell.py

@@ -1061,8 +1061,10 @@ class PyShell(OutputWindow):
                    (sys.version, sys.platform, self.COPYRIGHT, nosub))
         self.text.focus_force()
         self.showprompt()
+        # User code should use separate default Tk root window
         import tkinter
-        tkinter._default_root = None # 03Jan04 KBK What's this?
+        tkinter._support_default_root = True
+        tkinter._default_root = None
         return True
 
     def stop_readline(self):