The former indirect test did pass even if the exception have not been
raised.

These have been missing the "progname" parameter. Anyhow, due to the
nature of the test, this did not show up.

Explicitly require a first argument called "progname". This is to
avoid the common error not passing a progname by pointing users to it.
This is fully backward compatible.

This is to make programmers aware of the common error not passing a
progname.

Former code did add the progname to the list of arguments, which
somehow did hide it.  New code passes it when calling
ghostscript.Ghostscript(), thus making it more explicit.

Accoding to documentation, to enable user_errors, a negative value has
to be passed. "c_int(True)" is a positive value, which is treated the
same as zero (False).

The code based on th C-code did not work, since _gsprint raises
exceptions.

Instead it aises an exception like for any other error except e_Info.
This only effects the low-level API, the high-level API was adjusted.

… interface only. There are confusion about this by a user.

As recommended by the vendor.

Depending on the platform ghostscript might use different fonts or
different versions of the font. Thus switch to test images being
independent of fonts, using just some geometric forms.

Thanks to Elena ``of Valhalla'' for reporting.

/closes #27

Use SPDX license expression in license field.
Remove license classifier field, which are deprecated.

This was used for building sdist and wheel. Now use pypa/build instead.

Remove the version numbers from the python environment names and use
just "py3". This will make the test being run by the default python3
installed.

Drawback: This will no longer by default test with other Python
versions installed on a system. Anyhow, it is unknown whether anybody
actually uses this feature for this project and it removes some
maintenance burden.

Remove code for handing Python 2.