Update packagist description; test with php 7.3

Created by: TysonAndre

The description on https://packagist.org/packages/ulrichsg/getopt-php is misleading, the tests pass locally in php 7.3

The only issues in php 7.4 were

• #146
• Deprecation notice due to each() in phpunit (not getopt). each() was deprecated in php 7.3 and will be removed in php 8.0