Haystack's Elasticsearch 7 backend has a bug where boolean values were

being serialized as True or False. Starting in Elasticsearch 7,

these must explicitly be true or false.
We ship copies of the Elasticsearch backends, originally intended as

backports for Review Board 4, but this gives us the opportunity to work

around this in our end. Since Haystack hasn't had a release in a year,

and this bug was reported to them about that long ago, we can't rely on

a version bump any time soon.
This change patches our copy of the Elasticsearch 7 backend to properly

serialize booleans.
The Haystack bug report is available at:
https://github.com/django-haystack/django-haystack/issues/1851

Set up Elasticsearch 7 and triggered the bug. Verified that the fix

allowed me to properly set up a search involving booleans.