$ qlist python | grep pystrhex.h /usr/include/python3.5m/pystrhex.h /usr/include/python3.6m/pystrhex.h Looks like the file is present since py3.5. However, it's missing on pypy3 which pretends being py3.5 at the moment. As a result, dev-python/pysha3 fails to build on pypy3 because of the missing header file.
$ qlist python | grep pystrhex.h /usr/include/python3.10/pystrhex.h /usr/include/pypy3.10/pystrhex.h