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

* 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.

Lib/tkinter/simpledialog.py

@@ -24,9 +24,7 @@ askstring -- get a string from the user
 """
 
 from tkinter import *
-from tkinter import messagebox
-
-import tkinter # used at _QueryDialog for tkinter._default_root
+from tkinter import messagebox, _get_default_root
 
 
 class SimpleDialog:
@@ -128,13 +126,17 @@ class Dialog(Toplevel):
 
             title -- the dialog title
         '''
-        Toplevel.__init__(self, parent)
+        master = parent
+        if not master:
+            master = _get_default_root('create dialog window')
+
+        Toplevel.__init__(self, master)
 
         self.withdraw() # remain invisible for now
-        # If the master is not viewable, don't
+        # If the parent is not viewable, don't
         # make the child transient, or else it
         # would be opened withdrawn
-        if parent.winfo_viewable():
+        if parent is not None and parent.winfo_viewable():
             self.transient(parent)
 
         if title:
@@ -155,7 +157,7 @@ class Dialog(Toplevel):
 
         self.protocol("WM_DELETE_WINDOW", self.cancel)
 
-        if self.parent is not None:
+        if parent is not None:
             self.geometry("+%d+%d" % (parent.winfo_rootx()+50,
                                       parent.winfo_rooty()+50))
 
@@ -259,9 +261,6 @@ class _QueryDialog(Dialog):
                  minvalue = None, maxvalue = None,
                  parent = None):
 
-        if not parent:
-            parent = tkinter._default_root
-
         self.prompt   = prompt
         self.minvalue = minvalue
         self.maxvalue = maxvalue