bpo-32726: macOS 10.6+ installer now builds Tcl/TK 8.6 (GH-6307)

Build and link with private copy of Tcl/Tk 8.6 for the macOS 10.6+
installer. The 10.9+ installer variant already does this.  This means that
the Python 3.7 provided by the python.org macOS installers no longer need or
use any external versions of Tcl/Tk, either system-provided or user-
installed, such as ActiveTcl.
diff --git a/Mac/BuildScript/resources/ReadMe.rtf b/Mac/BuildScript/resources/ReadMe.rtf
index aecbaa3..81d4a99 100644
--- a/Mac/BuildScript/resources/ReadMe.rtf
+++ b/Mac/BuildScript/resources/ReadMe.rtf
@@ -53,16 +53,10 @@
 \f0  included with this installer has its own default certificate store for verifying download connections.\
 \
 
-\b \ul Using IDLE or other Tk applications [NEW/CHANGED in 3.7.0b1] 
+\b \ul Using IDLE or other Tk applications [NEW/CHANGED in 3.7.0b3] 
 \b0 \ulnone \
 \
-The 10.9+ installer variant comes with its own private version of Tcl/Tk 8.6. It does not use system-supplied or third-party supplied versions of Tcl/Tk.\
-\
-For the 10.6+ variant, you continue to need to install a newer third-party version of the 
-\i Tcl/Tk
-\i0  8.5 (not 8.6) frameworks to use IDLE or other programs that use the Tkinter graphical user interface toolkit.  Visit {\field{\*\fldinst{HYPERLINK "https://www.python.org/download/mac/tcltk/"}}{\fldrslt https://www.python.org/download/mac/tcltk/}} for current information about supported and recommended versions of 
-\i Tcl/Tk
-\i0  for this version of Python and of macOS.\
+Both installer variants come with their own private version of Tcl/Tk 8.6. They no longer use system-supplied or third-party supplied versions of Tcl/Tk.\
 
 \b \ul \
 Other changes\