_get_file: fix makedirs call when the lpath is relative and in the working directory

[skshetry]

Let's say if lpath is file literal, a relative path and to be downloaded to
the current working directory, _get_file currently fails with FileNotFoundError
during os.makedirs call.

await fs._get_file('gs://bucket/file', 'file')

This happens because os.path.dirname() returns an empty string, which when passed to os.makedirs(),
it raises a FileNotFoundError.

>>> os.path.dirname('file')
''
>>> os.makedirs('')
FileNotFoundError: [Errno 2] No such file or directory: ''

The following is a part of a traceback that I get.

File "/Users/user/dev/solutions/checkpoints-gcp/.venv/lib/python3.11/site-packages/fsspec/asyn.py", line 118, in wrapper
    return sync(self.loop, func, *args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/Users/user/dev/solutions/checkpoints-gcp/.venv/lib/python3.11/site-packages/fsspec/asyn.py", line 103, in sync
    raise return_result
File "/Users/user/dev/solutions/checkpoints-gcp/.venv/lib/python3.11/site-packages/fsspec/asyn.py", line 56, in _runner
    result[0] = await coro
                ^^^^^^^^^^
File "/Users/user/dev/solutions/checkpoints-gcp/.venv/lib/python3.11/site-packages/gcsfs/core.py", line 1491, in _get_file
    await self._get_file_request(u2, lpath, callback=callback, **kwargs)
File "/Users/user/dev/solutions/checkpoints-gcp/.venv/lib/python3.11/site-packages/decorator.py", line 221, in fun
    return await caller(func, *(extras + args), **kw)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/Users/user/dev/solutions/checkpoints-gcp/.venv/lib/python3.11/site-packages/gcsfs/retry.py", line 123, in retry_request
    return await func(*args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/Users/user/dev/solutions/checkpoints-gcp/.venv/lib/python3.11/site-packages/gcsfs/core.py", line 1472, in _get_file_request
    os.makedirs(os.path.dirname(lpath), exist_ok=True)
File "<frozen os>", line 225, in makedirs

FileNotFoundError: [Errno 2] No such file or directory: ''

The fix here is to fallback to os.curdir on an empty string.

[skshetry]

I used tempdir to be consistent with the other tests, but we could simplify this using tmp_path fixture and use monkeypatch.chdir() on it.

Suggested change

	with tempdir() as dn:
	os.makedirs(dn)
	monkeypatch.chdir(dn)
	monkeypatch.chdir(tmp_path)