please add compatibility with netresearch/jsonmapper ^3 and ^4

[umesecke]

There are two new major versions of netresearch/jsonmapper which as far as I can see only break BC by dropping support for old versions of PHP and PHPUnit.

Right now this package's constraint on versions ^1 or ^2 seems to stop us from updating our project to the current version of jsonmapper (through vimeo/psalm 🤷).

[github-actions]

🎉 This issue has been resolved in version 3.2.1 🎉

The release is available on GitHub release

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