In newer versions of https://github.com/esnme/ultrajson, ujson does not
serialize frozendicts (introduced in esnme/ultrajson@53f85b1). Although
the PyPI version is still 1.35, Fedora ships with a build from commit
esnme/ultrajson@2f1d487. This causes the serialization to fail if the
distribution-provided package is used.
This runs the event through the unfreeze utility before serializing it.
Thanks to @ignatenkobrain for tracking down the root cause.
fixes#2351
Signed-off-by: Jeremy Cline <jeremy@jcline.org>