setup: Allow to easily install test requirements

setup.py

@@ -20,6 +20,12 @@
 from setuptools import setup, find_packages
 import versioneer
 
+tests_require = [
+    'capturer',
+    'urllib3 >= 1.18',  # https://github.com/urllib3/urllib3/issues/258
+    'ipaddress',
+]
+
 setup(
   name='kedifa',
   version=versioneer.get_version(),
@@ -49,11 +55,8 @@ setup(
                 # is not working in some cases, but fortunately KeDiFa is
                 # used in places with pinned versions
   ],
-  tests_require=[
-    'capturer',
-    'urllib3 >= 1.18',  # https://github.com/urllib3/urllib3/issues/258
-    'ipaddress',
-  ],
+  tests_require=tests_require,
+  extras_require={'test': tests_require},
   zip_safe=True,
   entry_points={
     'console_scripts': [