[hailtop.batch] eliminate unittest; use single, session-wide event loop

[danking]

Fixes #14130.

We pervasively assume:

1. That our entire system is used within a single Python thread.
2. That once an event loop is created that's the only event loop that will exist forever.

Pytest (and newer version of IPython, afaict) violate this pretty liberally.

pytest_asyncio has explicit instructions on how to run every test in the same event loop. I've implemented those here. These instructions don't work. It seems that the reliable way to ensure we're using one event loop everywhere is to use pytest-asyncio < 0.23 and to define an event_loop fixture with scope 'session'.

I also switched test_batch.py into pytest-only style. This allows me to use session-scoped fixtures so that they exist exactly once for the entire test suite execution.

Also:

• RouterAsyncFS methods must either be a static method or an async method. We must not create an FS in a sync method. Both parse_url and copy_part_size now both do not allocate an FS.
• httpx.py now eagerly errors if the running event loop in request differs from that at allocation time. Annoying but much better error message than this nonsense about timeout context managers.
• hail_event_loop either gets the current thread's event loop (running or not, doesn't matter to us) or creates a fresh event loop and sets it as the current thread's event loop. The previous code didn't guarantee we'd get an event loop b/c get_event_loop fails if set_event_loop was previously called.
• conftest.py is inherited downward, so I lifted fixtures out of test_copy.py and friends and into a common hailtop/conftest.py
• I added make -C hail pytest-inter-cloud for testing the inter cloud directory. You still need appropriate permissions and authn.
• I removed extraneous pytest.mark.asyncio since we use auto mode everywhere.
• FailureInjectingClientSession creates an aiohttp.ClientSession and therefore must be used while an event loop is running. Easiest fix was to make the test async.

[daniel-goldstein]

@danking Looks like we need to upgrade pytest_asyncio

[danking]

@daniel-goldstein ok it finally works.