Port gsettings-schema-convert to python3

This is a straight-up 2to3 -w gsettings-schema-convert plus a modification of the shebang.

Other patches exist out there to fix some of the syntax errors for this file on Python 3 (see debian), but are incomplete - the resulting files still contain syntax errors on Python 3 in their print statements, and use some missing methods on dicts.

gsettings-schema-convert is not a complicated script, and as a seasoned porter of Python 2 code to Python 3, the 2to3 changes look straightforward, I think there is a good chance the script is correct without further changes.

Since this is an obsolete project I'm not sure if there's much chance this will be merged, but distros might want to ship this patch in order to drop Python 2 from their dependencies when it is EOL.