bpo-37323: Suppress DeprecationWarning raised by @asyncio.coroutine (GH-14293)

When the test is ran with `PYTHONWARNINGS=error` the environment variable is passed to the python interpreter used in `assert_python_ok` where `DeprecationWarning` from  `@asyncio.coroutine` is converted into an error. Ignore the `DeprecationWarning` in `assert_python_ok`.


https://bugs.python.org/issue37323

Lib/test/test_asyncio/test_tasks.py

@@ -3352,7 +3352,9 @@ class CompatibilityTests(test_utils.TestCase):
 
             asyncio.run(old_style_coro())
         """)
-        assert_python_ok("-c", code, PYTHONASYNCIODEBUG="1")
+
+        assert_python_ok("-Wignore::DeprecationWarning", "-c", code,
+                         PYTHONASYNCIODEBUG="1")
 
 
 if __name__ == '__main__':