setup.py: add "testing" extras_require

Ref: https://github.com/davidhalter/parso/issues/15#issuecomment-339964845

setup.py

@@ -46,4 +46,9 @@ setup(name='parso',
           'Topic :: Text Editors :: Integrated Development Environments (IDE)',
           'Topic :: Utilities',
       ],
+      extras_require={
+          'testing': [
+              'pytest>=3.0.7',
+          ],
+      },
       )