Update jsonschema to 3.0.1

[pyup-bot]

This PR updates jsonschema from 2.6.0 to 3.0.1.

Changelog

3.0.0

------

* Support for Draft 6 and Draft 7
* Draft 7 is now the default
* New ``TypeChecker`` object for more complex type definitions (and overrides)
* Falling back to isodate for the date-time format checker is no longer
attempted, in accordance with the specification

Links

• PyPI: https://pypi.org/project/jsonschema
• Changelog: https://pyup.io/changelogs/jsonschema/
• Repo: https://github.com/Julian/jsonschema

[pyup-bot]

Closing this in favor of #246