Loosen requirement on `jsonschema`

[morincl]

Any Python project depending on darwin-py and jsonschema simultaneously is forced to use jsonschema==3.2.0 if the version is pinned by darwin-py. This is causing trouble as my project requires jsonschema 4, which is anyways backwards compatible with jsonschema 3 and therefore should not be a problem for darwin-py.

Adding jsonschema<5.0 allows to avoid potential future trouble from breaking changes the next time that jsonschema makes a major release.

[rslota]

@morincl Thanks for the PR. Unfortunately this update is not that straightforward due to 2 reasons:

1. jsonschema 4.x is not supported on python 3.6 which darwin-py supports. This can be easily fixed by pinning the version conditionally.
2. jsonschema 4.x is not backwards compatible if you consider validation error formatter. We have a bunch of tests that are failing on newer versions. I think it would be fine if we dumped the version to 4.x but there are tests that need to be adjusted.

[rmax]

@rslota is there any chance this is going to be fixed before end of year?

[owencjones]

@rmax There is a ticket currently going through QA and then merge that will fix this, and address some other issues.

[rmax]

@owencjones thanks! This is awesome