I would enforce it. For example, constructing CleintSession in a module global namespace is a bad smell and leads to hard-to-debug errors very often.

The plan is:

1. Add helpers.get_running_loop() function with the following implementation:

def get_running_loop(loop=None):
    if loop is None:
        loop = asyncio.get_event_loop()
    if not loop.is_running():
        warnings.warn("The object should be created from async function",
                                 DeprecationWarning,
                                 stacklevel=3)

2. Replace asyncio.get_event_loop() with get_running_loop() in all aiohttp objects.
3. Replace a deprecation warning with RuntimeError() in aiohttp 4.0