[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/tkinter/test/test_tkinter/test_variables.py

@@ -1,8 +1,10 @@
 import unittest
 import gc
+import tkinter
 from tkinter import (Variable, StringVar, IntVar, DoubleVar, BooleanVar, Tcl,
                      TclError)
 from test.support import ALWAYS_EQ
+from tkinter.test.support import AbstractDefaultRootTest
 
 
 class Var(Variable):
@@ -308,8 +310,21 @@ class TestBooleanVar(TestBase):
             v.get()
 
 
+class DefaultRootTest(AbstractDefaultRootTest, unittest.TestCase):
+
+    def test_variable(self):
+        self.assertRaises(RuntimeError, Variable)
+        root = tkinter.Tk()
+        v = Variable()
+        v.set("value")
+        self.assertEqual(v.get(), "value")
+        root.destroy()
+        tkinter.NoDefaultRoot()
+        self.assertRaises(RuntimeError, Variable)
+
+
 tests_gui = (TestVariable, TestStringVar, TestIntVar,
-             TestDoubleVar, TestBooleanVar)
+             TestDoubleVar, TestBooleanVar, DefaultRootTest)
 
 
 if __name__ == "__main__":