[ANNOUNCE] dtschema 2021.5 release

I don't normally announce releases, but I've tagged a new dtschema
release. There's exactly one new feature in this release: improved
warning messages for binding schema warnings.

The default error messages come from the jsonschema module. For
anything beyond simple errors, figuring out where in the meta-schemas
the error came from and how the schema needs to be changed are not
obvious. This should now be much better as the nearest 'description'
in the meta-schema and the '$id' of the failing meta-schema will be
printed. I've added some descriptions which cover the most common
cases. I'm sure there's still more descriptions to add, so let me know
or send me patches.

The release is available on PyPI: https://pypi.org/project/dtschema/

Rob