From #3742 (comment):

Hello! Thank you for this great library! The Mozilla release engineering team relies on it for several projects 😃

I just found a similar regression:

AttributeError: 'async_generator' object has no attribute 'put'

This async generator is a regular aiohttp.ClientSession() and is initialized here. This might be an integration issue with the pytest-asyncio plugin.

I rolled pytest back to 3.6.4 and things are back working. You can see both the failure and the workaround in

• Fix fake_session breakage mozilla-releng/beetmoverscript#176
• https://travis-ci.org/mozilla-releng/beetmoverscript/jobs/410250756#L598

Example

I put up a minimal example over here: https://github.com/JohanLorenzo/pytest-fixture-bug-3747