Copy files from FTP to an ObjectStore

[JackKelly]

This PR implements a simple, concurrent strategy for copying many files from an FTP server to an ObjectStore.

The main use-case for this code is copy GRIB files from DWD's FTP server to dynamical's source co-op bucket (see issue #258). To keep this PR small(ish), the code in this PR doesn't know anything about DWD. This PR just implements a general-purpose utility for downloading many FTP files.

A follow-on PR will implement logic to find which files to download.

Note that this PR relies on PR #319 to bump the Python version from 3.12 to 3.13 so we can use asyncio.Queue.shutdown (which was only introduced in 3.13). Once PR #319 is merged into main, this current PR will only show 3 files as changed.

Still TODO before this PR is ready for review:

• Implement retry logic in obstore_worker.
• Implement mock FTP server for testing.
• Save to a temporary directory for testing.
• Fix Ruff failures in main code due to updating to Python 3.13 (see comment below) (implemented in PR chore: upgrade to Python 3.13 and fix linting errors #319)
• Convert print statements to log statements.
• Or, actually, maybe don't return any list of files, but just log failures. And rely on the code that figures out what files to download to find missing files.
• Describe function params in docstrings.
• Tidy up log statements: remove some; change log levels on some.
• Test failed FTP connection.
• Test failed obstore write.
• Test missing FTP file.

[JackKelly]

@aldenks would you be comfortable with me adding pytest-asyncio to the dev dependencies in pyproject.toml? It's not a biggie. We can test async functions without pytest-asyncio, it's a just a little prettier with pytest-asyncio:

Without pytest-asyncio:

@pytest.fixture
def sleep_time() -> float:
    return 0.5

async def _foo(sleep_time: float):
    await asyncio.sleep(sleep_time)

def test_foo(sleep_time: float):
    # Fixtures aren't applied to helper functions, so we have to pass in `sleep_time`:
    asyncio.run(_foo(sleep_time))

With pytest-asyncio

And after setting asyncio_mode = "auto" in [tool.pytest] in pyproject.toml:

@pytest.fixture
def sleep_time() -> float:
    return 0.5

async def test_foo(sleep_time: float):
    await asyncio.sleep(sleep_time)

[aldenks]

Nice, and thanks for breaking up into manageable PRs! you're welcome to add pytest-asyncio to dev deps

My comments are all small, merge to main yourself when you've addressed whichever of them make sense