Fix async test

Author: bgreen-litl

tests/test_backoff_async.py

@@ -585,14 +585,14 @@ def lookup_max_tries():
     @backoff.on_exception(backoff.constant,
                           ValueError,
                           max_tries=lookup_max_tries)
-    def exceptor():
+    async def exceptor():
         raise ValueError()
 
     with pytest.raises(ValueError):
-        exceptor()
+        await exceptor()
 
     with pytest.raises(ValueError):
-        exceptor()
+        await exceptor()
 
     assert len(lookups) == 2