We just taught the Windows installers to produce Python 3 variants
built with PyOxidizer.
Our plan is to publish both Python 2 and Python 3 versions of the
installers for Mercurial 5.4.
This commit teaches the Inno and WiX installers to add an optional
string suffix to the installer name. On Python 2, that suffix is
"-python2." We reserve the existing name for the Python 3 installers,
which we want to make the default.