requirements: Pin jsonschema

requirements: Pin jsonschema

The new 4.x release of jsonschema has borked our schemas for reasons I
do not yet fully understand (though I have an idea - see inline). Unbork
things now and fix our CI, while we work on a longer-term fix.

Conflicts:
  requirements-test.txt

Changes:
  requirements-text.txt

NOTE(stephenfin): The conflict is because we're using a slightly older
version of openapi-core here which we don't want to update. We also need
to modify the file futher to pin openapi-schema-validator to keep pip
happy since openapi-schema-validator 0.2.0 requires jsonschema > 4.0.

Signed-off-by: Stephen Finucane <stephen@that.guru>