Switch to dnspython from dnspython3

Codebase has been unified, see issue #325

Author: PromyLOPh

setup.py

@@ -104,7 +104,7 @@ def get_version():
 
 setup_kwargs['install_requires'] = [
     'chardet>=2.0.1,<=2.3',
-    'dnspython3==1.12',
+    'dnspython>=1.13',
     'html5lib>=0.999,<=0.9999999',
     'lxml>=3.1.0,<=3.5',
     'namedlist>=1.3,<=1.7',