openSourceUrl($url) must be compatible

Problem/Motivation

Cannot run example migration in Php 8.

ddev drush ms --group=migration_tools_example

PHP Fatal error: Declaration of Drupal\migration_tools\Plugin\migrate_plus\data_parser\Dom::openSourceUrl($url) must be compatible with Drupal\migrate_plus\DataParserPluginBase::openSourceUrl(string $url): bool in /var/www/html/docroot/modules/contrib/migration_tools/src/Plugin/migrate_plus/data_parser/Dom.php on line 93

Steps to reproduce

* enable example module
* run above code
* confirm error

Proposed resolution

* Add return type to DOM class

Remaining tasks

User interface changes

API changes

Data model changes