[3.8] bpo-42630: Improve error reporting in Tkinter for absent default root (GH-23781...

[3.8] bpo-42630: Improve error reporting in Tkinter for absent default root (GH-23781) (GH-23854)

* 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 3d569fd6dccf9f582bafaca04d3535094cae393e)