uasyncio.core: async() was renamed to ensure_future() upstream.

The old name is deprecated since CPyton 3.4.4. That's great relief, because
CPython has special parser hacks to support both 3.5-style "async" keyword,
and still allow it be used as an identifier in other contexts. MicroPython
lacks such hacks, so "async" is SyntaxError as a function name with the latest
versions which implement async keyword.

uasyncio.core/uasyncio/core.py

@@ -167,7 +167,7 @@ def coroutine(f):
 # for compatibility with CPython asyncio
 #
 
-def async(coro, loop=_event_loop):
+def ensure_future(coro, loop=_event_loop):
     _event_loop.call_soon(coro)
     # CPython asyncio incompatibility: we don't return Task object
     return coro