I thought to have replied to the list, but it turns out it was a private conversation
only.
Forwarding Mikolaj's answer to anyone interested, example package in question was
concurrentunit.
How to fix?
fedpkg clone concurrentunit
cd concurrentunit
fedpkg srpm
mock --enable-plugin pm_request concurrentunit-0.4.2-2.fc25.src.rpm
xmvn-builddep
This prints build-requires on stdout. Just copy these to spec file.