S3 list operation requested when file not found

What happened:
When trying to open a file that does not exist in an AWS S3 bucket, a list operation is requested. In addition, a FileNotFoundError exception is raised.

What you expected to happen:
I would expect to see a FileNotFoundError exception being raised only. No list operations should be requested. It seems unnecessary and AWS S3 charges for it.

Minimal Complete Verifiable Example:
When running:

import s3fs


storage_options = {
    'key': 'my-key',
    'secret': 'my-secret',
    'use_ssl': False,
    'client_kwargs': {
        'endpoint_url': 'http://my-url:my-port'
    },
}
file_system = s3fs.S3FileSystem(**storage_options)
file_system.open('s3://my-bucket/my_file.txt', 'r')

The output is:

FileNotFoundError: my-bucket/my_file.txt

And my HTTP traffic monitor shows the GET request for the list operation (note the list-type=2):

GET     /my-bucket?list-type=2&prefix=my_file.txt%2F&delimiter=%2F&max-keys=1&encoding-type=url

Anything else we need to know?:
The list operation seems to be requested when this line is executed.

Environment:

• Python version: 3.6.9
• Operating System: Ubuntu 18.04.3 LTS
• Install method (conda, pip, source): pip install s3fs
• Output of pip list:

Package           Version
----------------- -------
aiobotocore       1.1.2
aiohttp           3.6.2
aioitertools      0.7.0
async-timeout     3.0.1
attrs             20.2.0
botocore          1.17.44
chardet           3.0.4
docutils          0.15.2
fsspec            0.8.3
idna              2.10
idna-ssl          1.1.0
jmespath          0.10.0
multidict         4.7.6
pip               20.2.3
pkg-resources     0.0.0
python-dateutil   2.8.1
s3fs              0.5.1
setuptools        50.3.0
six               1.15.0
typing-extensions 3.7.4.3
urllib3           1.25.10
wheel             0.35.1
wrapt             1.12.1
yarl              1.6.0

Start with the referenced fsspec/spec.py lines 1212-1213 and reproduce the S3FileSystem.open example while monitoring requests. Trace why the missing-file path performs a list operation, then verify that it raises FileNotFoundError without issuing that request.