test: allow aiohttp to leak HTTPS connections

[jpivarski]

Because of aio-libs/aiohttp#1115, the aiohttp library can let an HTTPS connection dangle for one event longer than pytest or Python allows, triggering an "unclosed transport" ResourceError.

We do want to be sticklers for open file handles in our tests, so the filterwarnings entry for this was made very narrow, to only include this case.

@lobis, did you know about this? Is the alternative of having FSSpecSource do

await asyncio.sleep(0)

when it goes out of a context manager viable?