Change minimun version for dnspython due to deprecation warning

setup.py

@@ -32,7 +32,7 @@ try:
     import DNS
     kw['install_requires'] = ['Py3DNS']
 except ImportError:  # If PyDNS is not installed, prefer dnspython
-    kw['install_requires'] = ['dnspython>=1.16.0']
+    kw['install_requires'] = ['dnspython>=2.0.0']
 
 with open("README.md", "r") as fh:
     long_description = fh.read()