Replace pathfix with py3_shebang_fix
    
    %pyproject_install currently has:
    `pathfix%{python3_version}.py -pni "%{__python3}" -k%{?py3_shbang_opts: -a%{py3_shbang_opts_nodash}} %{buildroot}%{_bindir}/*`
    
    We should replace it with `%py3_shebang_fix %{buildroot}%{_bindir}/*` which expands to:
    ~~~~
      if [ -f /usr/bin/pathfix%{python3_version}.py ]; then
        pathfix=/usr/bin/pathfix%{python3_version}.py
      else
        # older versions of Python don't have it and must BR /usr/bin/pathfix.py from python3-devel explicitly
        pathfix=/usr/bin/pathfix.py
      fi
      $pathfix -pni %{__python3} -k%{?py3_shebang_flags:a %py3_shebang_flags}} %{buildroot}%{_bindir}/*
    ~~~~
    
    Mainly so that we:
     - switch to %py3_shebang_flags
     - have only one place to fix the invocation
    
    Fixes: rhbz#1868347