Dependency to jsonschema is really old

[MartinWallgren]

Description

It's not possible to use this lib if you have requirements to jsonschema >=4.0.0.
The release date for jsonschema 4.0.1 was Sep 30, 2021.

Motivation

17 months is plenty of time to upgrade, and your clients might need software that relies on jsonschema 4.

Exemplification

Package A depends on jsonschema >4.0.0

Package B depends on package A

Package B also want to use eiffellib, but that is impossible due to jsonschema verision constraints.

Benefits

More people can use this lib.

Possible Drawbacks

jsonschema >4 might not be compatible.

[magnusbaeck]

We should close this issue since #55 has been merged, right?