bpo-33479: Remove unqualified tkinter threadsafe claim. (GH-6990) (GH-27705)

It has not been true for several years and likely never was.
(cherry picked from commit 6b37d0d5300813de31d66df1c77dad7e1027e4d8)

Co-authored-by: Terry Jan Reedy <tjreedy@udel.edu>

Doc/library/tk.rst

@@ -19,8 +19,7 @@ The :mod:`tkinter` package is a thin object-oriented layer on top of Tcl/Tk. To
 use :mod:`tkinter`, you don't need to write Tcl code, but you will need to
 consult the Tk documentation, and occasionally the Tcl documentation.
 :mod:`tkinter` is a set of wrappers that implement the Tk widgets as Python
-classes.  In addition, the internal module :mod:`_tkinter` provides a threadsafe
-mechanism which allows Python and Tcl to interact.
+classes.
 
 :mod:`tkinter`'s chief virtues are that it is fast, and that it usually comes
 bundled with Python. Although its standard documentation is weak, good

Misc/NEWS.d/next/Documentation/2018-05-19-15-59-29.bpo-33479.4cLlxo.rst

@@ -0,0 +1,4 @@
+Remove the unqualified claim that tkinter is threadsafe. It has not been
+true for several years and likely never was. An explanation of what is true
+may be added later, after more discussion, and possibly after patching
+_tkinter.c,