bpo-42560: tweaks to intro of Tkinter docs- add macOS, drop ActiveState (GH-27835)

(cherry picked from commit 141c7a4) Co-authored-by: Mark Roseman <mark@markroseman.com>

Author: miss-islington

Doc/library/tkinter.rst

@@ -11,9 +11,8 @@
 --------------
 
 The :mod:`tkinter` package ("Tk interface") is the standard Python interface to
-the Tk GUI toolkit. Both Tk and :mod:`tkinter` are available on most Unix
-platforms, as well as on Windows systems. (Tk itself is not part of Python; it
-is maintained at ActiveState.)
+the Tcl/Tk GUI toolkit. Both Tk and :mod:`tkinter` are available on most Unix
+platforms, including macOS, as well as on Windows systems.
 
 Running ``python -m tkinter`` from the command line should open a window
 demonstrating a simple Tk interface, letting you know that :mod:`tkinter` is