The rpm-build RPM started to require Python 2 since version 4.14.1-2. This happened because `python-macro-helper` script was added to the package. The script has #!/usr/bin/python shebang which generates this dependency. Can it be switched to #!/usr/bin/python3?
The script actually doesn't need a shebang because it's never invoked as `.../python-macro-helper` but always as `%{__python} ... .../python-macro-helper`. I reported that upstream already as https://github.com/rpm-software-management/rpm/issues/387 However here in Fedora this pulls python2 into the default buildroot once again, so the problem is more severe.
https://src.fedoraproject.org/rpms/rpm/pull-request/11
Fixed in rpm-4.14.1-6.fc28