skf-2.10.12-1.fc33.3 fails to build in Fedora 33, because a SRPM package cannot built:

DEBUG util.py:719:  Executing command: ['rpmbuild', '--define', '_topdir /chroot_tmpdir/srpm_unpacked', '-bs', '--nodeps', '/chroot_tmpdir/srpm_unpacked/SPECS/skf.spec'] with env {'TERM': 'vt100', 'SHELL': '/bin/bash', 'HOME': '/builddir', 'HOSTNAME': 'mock', 'PATH': '/usr/bin:/bin:/usr/sbin:/sbin', 'PROMPT_COMMAND': 'printf "\\033]0;<mock-chroot>\\007"', 'PS1': '<mock-chroot> \\s-\\v\\$ ', 'LANG': 'C.UTF-8'} and shell False
DEBUG util.py:448:  Unsharing. Flags: 134217728
DEBUG util.py:621:  error: attempt to use unversioned python, define %__python to /usr/bin/python2 or /usr/bin/python3 explicitly
DEBUG util.py:621:  error: /chroot_tmpdir/srpm_unpacked/SPECS/skf.spec: line 8: Macro %python_sitearch failed to expand
DEBUG util.py:774:  Child return code was: 1

A difference between a passing and failing build root is at <https://koschei.fedoraproject.org/build/8621153>. This is triggered by upgrading python-srpm-macros from 3.9-5 to 3.9-6:

* Thu Jul 16 2020 Miro Hrončok <mhroncok> - 3.9-6
- Make the unversioned %__python macro error
- https://fedoraproject.org/wiki/Changes/PythonMacroError