Fix wait_closed dependency on Python <3.7

[srh]

Describe the bug
#267 uses an API that was added in Python 3.7, StreamWriter's wait_closed method. Prior to that, asyncio went out of its way to work with Python 3.5 and 3.6 (while the async def changes broke 3.4 for compatibility with later Python versions). We should branch on Python version to minimize breakage for older users.