libselinux: Build selinux python package using setup.py
    
    Originally, setup.py built selinux as a module. It caused problems with SWIG
    generated python wrapper which used wrong path for importing binary module.
    
    selinux is built as a python package now.
    
    Fixes:
    >>> import selinux
    Traceback (most recent call last):
      File "/usr/lib64/python3.7/site-packages/selinux/__init__.py", line 14, in swig_import_helper
        return importlib.import_module(mname)
      File "/usr/lib64/python3.7/importlib/__init__.py", line 127, in import_module
        return _bootstrap._gcd_import(name[level:], package, level)
      File "<frozen importlib._bootstrap>", line 1006, in _gcd_import
      File "<frozen importlib._bootstrap>", line 983, in _find_and_load
      File "<frozen importlib._bootstrap>", line 965, in _find_and_load_unlocked
    ModuleNotFoundError: No module named '_selinux'