bpo-38329: python.org macOS installers now update Current symlink (GH-19650)

Previously, python.org macOS installers did not alter the Current version
symlink in /Library/Frameworks/Python.framework/Versions when installing
a version of Python 3.x, only when installing 2.x.  Now that Python 2 is
retired, it's time to change that.  This should make it a bit easier
to embed Python 3 into other macOS applications.