I am unsure if there is any Python 3.8 / Plone 5.2 parallel to what is described for latest setuptools in Plone 6.1.x here.
What I notice on PyPI is that plone.restapi == 7.9.0 is only available as a source distribution — I do not know if that helps narrow down what might be happening here.
I can say that I have been able to run plone.restapi = 8.37.0 in a Plone 5.2 on Python 3.8 without issue; this version has a built platform-agnostic wheel distribution on PyPI.