The copr-frontend (1.127-1.fc28) RPM requires both Python 2 and Python 3:
$ dnf --disablerepo=* --enablerepo=rawhide repoquery --requires copr-frontend | grep python
This happened due to recently introduced shebangs mangling , and you might see the warnings in the build log:
> *** WARNING: mangling shebang in /usr/share/copr/coprs_frontend/run/generate_repo_packages.py from #!/usr/bin/python to #!/usr/bin/python2. This will become an ERROR, fix it manually!
The executable contains `#!/usr/bin/env python` shebang, which is forbidden by the guidelines and is automatically mangled to `#!/usr/bin/python2` during build. Please fix the shebang to be `#!/usr/bin/python3` explicitly  or remove the executable bit from the file if not needed.
If anything is unclear, or if you need any kind of assistance, you can
ask on IRC (#fedora-python on Freenode), or reply here. We'll be happy
to help investigating or fixing this issue!
Hello, new COPR version has been released. This should be now fixed.