fix setuptools deprecation (#293)

Signed-off-by: mosfet80 <[email protected]>

Author: mosfet80

sensor_msgs_py/setup.py

@@ -19,5 +19,9 @@
     author_email='[email protected]',
     description='A package for easy creation and reading of PointCloud2 messages in Python.',
     license='BSD',
-    tests_require=['pytest'],
+    extras_require={
+        'test': [
+            'pytest',
+        ],
+    },
 )