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/commondialog.py

@@ -18,10 +18,10 @@ class Dialog:
     command = None
 
     def __init__(self, master=None, **options):
+        if not master:
+            master = options.get('parent')
         self.master = master
         self.options = options
-        if not master and options.get('parent'):
-            self.master = options['parent']
 
     def _fixoptions(self):
         pass # hook