fix(dependencies): require dataclasses package only for python versions <3.7

[skalish]

↪️ Pull Request

Avoids known bug when used in 3.8: https://github.com/mitsuse/typedjson-python/issues/8

✔️ PR Todo

• [ ] Testing for this change
• [ ] Links to related issues/PRs
• [ ] Update relevant docs + docstrings

[github-actions[bot]]

:tada: This PR is included in version 1.3.1 :tada:

The release is available on:

• v1.3.1
• GitHub release

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