Update jsonschema requirement from <4.18 to <4.21

Updates the requirements on [jsonschema](https://github.com/python-jsonschema/jsonschema) to permit the latest version.
- [Release notes](https://github.com/python-jsonschema/jsonschema/releases)
- [Changelog](https://github.com/python-jsonschema/jsonschema/blob/main/CHANGELOG.rst)
- [Commits](python-jsonschema/jsonschema@v0.1...v4.20.0)

---
updated-dependencies:
- dependency-name: jsonschema
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <support@github.com>

pyproject.toml

@@ -35,7 +35,7 @@ test = [
     'astropy >= 5.0.4',
     'gwcs',
     'packaging>=16.0',
-    'jsonschema<4.18',
+    'jsonschema<4.21',
 ]
 
 [project.urls]