Add pyproject-srpm-macros with a minimal %pyproject_buildrequires macro
    
    pyproject-srpm-macros is intended to be installed in the default buildroot.
    
    That way, no explicit BuildRequires for pyproject-rpm-macros are required,
    as long as %pyproject_buildrequires is used in %generate_buildrequires.
    
    When only pyproject-srpm-macros is installed, the minimal implementation of
    %pyproject_buildrequires generates a dependency on pyproject-rpm-macros.
    When pyproject-rpm-macros is installed, it overrides the implementation
    of %pyproject_buildrequires with the full one.
    
    Note that in Fedora, pyproject-rpm-macros is required by python3-devel,
    but not in RHEL.
    This allows us to keep pyproject-rpm-macros in the RHEL CRB repository.