Fix asyncio coroutine warning tests

[Mirochill]

Closes #18

Summary:

• Add an IS_PY34_ONLY test helper for the legacy asyncio.coroutine coverage.
• Limit the @asyncio.coroutine tests to Python 3.4, where async def is not available.
• Keep the existing async def coroutine tests for Python 3.5+.

Validation:

• git diff --check HEAD~1..HEAD
• Not run locally