Declare a [test] extra with test dependencies (#1074)

Since `python setup.py test` is deprecated and `tests_require` is only used by that,
this allows to programmatically read the tests dependencies from the metadata.

setup.py

@@ -59,6 +59,7 @@ if 'bdist_wheel' not in sys.argv:
 
 # bdist_wheel
 extras_require = {
+    'test':  tests_require,
     # https://wheel.readthedocs.io/en/latest/#defining-conditional-dependencies
     ':sys_platform == "win32"': install_requires_win_only,
 }