Add swig dependency

We don't want to download swig manually.
Hope pip install can finish it directly.

Signed-off-by: Gua Guo <gua.guo@intel.com>

setup.py

@@ -44,13 +44,13 @@ def run(self):
     name='pylibfdt',
     cmdclass = {'build_py' : build_py},
     use_scm_version=True,
-    setup_requires = ['setuptools_scm'],
+    setup_requires = ['setuptools_scm', 'swig'],
     author='Simon Glass <sjg@chromium.org>',
     description='Python binding for libfdt',
     ext_modules=[libfdt_module],
     py_modules=['libfdt'],
     package_dir={'': 'libfdt'},
-
+    python_requires=">=3.8",
     long_description=long_description,
     long_description_content_type="text/plain",
 )