Default to just calling python since that is most commonly available: the
official installer or zipfiles from python.org, anaconda, nupkg all result
in python being available but not python3. In other words: the default
used so far is wrong. Note that os.name is 'posix' when running the python
version which comes with Cygwin or MSys2 so they are not affected by this.
However of all possible ways to get Python on Windows, only Cygwin provides
no python command so update the default way for running tests in the
README.
0397448501