Improve item-piechart's matrix

What's Changed

• The JSON export should not export a placeholder item as an empty entry by @JasperCraeghs in #375
• Fix bug in error handling of invalid item-matrix options by @JasperCraeghs in #378
• Improvements for pie chart matrix by @JasperCraeghs in #379
• Modernize setup.py based build system and some refactor for pie-chart matrix headings by @JasperCraeghs in #380

Full Changelog: v11.2.0...v11.3.0