DOC: docstrings for Python version versions
NF: add README.txt file with install instructions

Install instructions for homebrew, macports, python.org Python.
RF: add python version to Makefile target

Check Python 3 install build works too.
NF: delocate wheels and add platform tags

Run delocate_wheels and add platform tags, in case we had to build any
wheels on the way.
NF+TST: add identifier property

Use in Distribution file.
RF: add title for installer via XML

Put title of installer into Distribution XML so it appears when
running the GUI.
RF: add list of installed packages for welcome

I'd forgotten to add the list of packages to install in the welcome page
of the installer.
DOC: add docstrings for PkgWriter

Fill out init and some other docstrings.
RF: remove package version from identifier

Package version stored separately from package identifier. Otherwise
upgrades of the same package will look like a new package.
RF+TST: add check for dmg file overwrite

Fail early in dmg write when dmg file exists.