Declare that types-jsonschema requires Python 3.8+

[AlexWaygood]

See #10772

[AlexWaygood]

Works like a charm. On Python 3.7:

> pip install "types-jsonschema>=4.19.0.2"
ERROR: Ignored the following versions that require a different python version: 4.19.0.2 Requires-Python >=3.8; 4.19.0.3 Requires-Python >=3.8
ERROR: Could not find a version that satisfies the requirement types-jsonschema>=4.19.0.2 (from versions: 3.2.0, 3.2.1, 4.2.0, 4.3.0, 4.3.1, 4.3.2, 4.4.0, 4.4.1, 4.4.2, 4.4.3, 4.4.4, 4.4.5, 4.4.6, 4.4.7, 4.4.8, 4.4.9, 4.6.0, 4.7.0, 4.9.0, 4.12.0, 4.12.1, 4.14.0, 4.15.0, 4.15.1, 4.16.0, 4.16.1, 4.16.1.1, 4.17.0.0, 4.17.0.1, 4.17.0.2, 4.17.0.3, 4.17.0.4, 4.17.0.5, 4.17.0.6, 4.17.0.7, 4.17.0.8, 4.17.0.9, 4.17.0.10, 4.19.0.0, 4.19.0.1)
ERROR: No matching distribution found for types-jsonschema>=4.19.0.2

Thanks again for #10724 and typeshed-internal/stub_uploader#103, @hamdanal!

[hamdanal]

Awesome to see all the pieces fall into place. Thank you Alex and also Sebastian for finishing the stub_uploader PR.

typeshed is one of the healthiest and most welcoming open source projects I've ever seen, you guys (all maintainers) are doing an amazing job 👏🏻.