3.1.0 - PHP 8.1 support

This release adds support for PHP 8.1, by addressing some new return types on internal interfaces. This does not adopt any 8.1 features and remains compatible with older versions.

What's Changed

• Update to PHPStan 1.x by @Firehed in #7
• Add PHP 8.1 to test suite by @Firehed in #8

Full Changelog: 3.0.0...3.1.0