Replace deprecated asyncio.iscoroutinefunction usage as a linter rule

[lmazuel]

All credit to @askpatrickw that created this issue:
Azure/msrest-for-python#265

Let's make sure the linter tag it, so all SDK can update accordingly. I still see a lot of usage:
https://github.com/search?q=repo%3AAzure%2Fazure-sdk-for-python%20asyncio.iscoroutinefunction&type=code