Fix py2-ipaddress dependency

Signed-off-by: Aanand Prasad <aanand.prasad@gmail.com>

setup.py

@@ -12,8 +12,9 @@ requirements = [
     'websocket-client >= 0.32.0',
 ]
 
-if sys.version_info[0] == 2:
-    requirements.append('py2-ipaddress >= 3.4.1')
+extras_require = {
+    ':python_version < "3"': 'py2-ipaddress >= 3.4.1',
+}
 
 exec(open('docker/version.py').read())
 
@@ -32,6 +33,7 @@ setup(
     ],
     install_requires=requirements,
     tests_require=test_requirements,
+    extras_require=extras_require,
     zip_safe=False,
     test_suite='tests',
     classifiers=[