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/test/test_tkinter/test_simpledialog.py

@@ -0,0 +1,25 @@
+import unittest
+import tkinter
+from test.support import requires, run_unittest, swap_attr
+from tkinter.test.support import AbstractDefaultRootTest
+from tkinter.simpledialog import Dialog, askinteger
+
+requires('gui')
+
+
+class DefaultRootTest(AbstractDefaultRootTest, unittest.TestCase):
+
+    def test_askinteger(self):
+        self.assertRaises(RuntimeError, askinteger, "Go To Line", "Line number")
+        root = tkinter.Tk()
+        with swap_attr(Dialog, 'wait_window', lambda self, w: w.destroy()):
+            askinteger("Go To Line", "Line number")
+        root.destroy()
+        tkinter.NoDefaultRoot()
+        self.assertRaises(RuntimeError, askinteger, "Go To Line", "Line number")
+
+
+tests_gui = (DefaultRootTest,)
+
+if __name__ == "__main__":
+    run_unittest(*tests_gui)