bpo-32726: macOS 10.6+ installer now builds Tcl/TK 8.6 (GH-6308)

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/build-installer.py b/Mac/BuildScript/build-installer.py
index 7f6228b..d2e88d8 100755
--- a/Mac/BuildScript/build-installer.py
+++ b/Mac/BuildScript/build-installer.py
@@ -4,7 +4,7 @@
 
 NEW for 3.7.0:
 - support Intel 64-bit-only () and 32-bit-only installer builds
-- use external Tcl/Tk 8.6 for 10.9+ builds
+- build and use internal Tcl/Tk 8.6 for 10.6+ builds
 - deprecate use of explicit SDK (--sdk-path=) since all but the oldest
   versions of Xcode support implicit setting of an SDK via environment
   variables (SDKROOT and friends, see the xcrun man page for more info).
@@ -24,7 +24,9 @@
 so will fetch them from PyPI if necessary.  Since python3 is now used for
 Sphinx, build-installer.py should also be converted to use python3!
 
-build-installer currently requires an installed third-party version of
+For 3.7.0, when building for a 10.6 or higher deployment target,
+build-installer builds and links with its own copy of Tcl/Tk 8.6.
+Otherwise, it requires an installed third-party version of
 Tcl/Tk 8.4 (for OS X 10.4 and 10.5 deployment targets), Tcl/TK 8.5
 (for 10.6 or later), or Tcl/TK 8.6 (for 10.9 or later)
 installed in /Library/Frameworks.  When installed,
@@ -190,9 +192,9 @@
 EXPECTED_SHARED_LIBS = {}
 
 # Are we building and linking with our own copy of Tcl/TK?
-#   For now, do so if deployment target is 10.9+.
+#   For now, do so if deployment target is 10.6+.
 def internalTk():
-    return getDeptargetTuple() >= (10, 9)
+    return getDeptargetTuple() >= (10, 6)
 
 # List of names of third party software built with this installer.
 # The names will be inserted into the rtf version of the License.