* Adding aliases for field attributes and renaming async attribute
As of Python 3.7, the use of async raises an error, whereas before the use
of the reserved word was ignored. This adds an alias field for field attrs
so that both async and async_val (interally) work. This allows us to be
backwards-compatible with 3rd party plugins that may still reference Task.async,
but for the core engine to work on Py3.7+.
* Remove files fixed for 'async' usage from the python 3.7 skip list
