put(recursive=True) gives IsADirectoryError when uploading to root of a bucket

[jherman3]

import s3fs

s3 = s3fs.S3FileSystem(client_kwargs={"endpoint_url": "http://my-minio-server.url"})
# test_bug is a folder with a few files in it. 
s3.put("/path/to/test_bug", "s3://bug", recursive=True)
print("Done")

This gives

Output:
Traceback (most recent call last):
  File "s3fs_bug.py", line 6, in <module>
    s3.put("/path/to/test_bug", "s3://bug", recursive=True)
  File "/snip/lib/python3.8/site-packages/fsspec/asyn.py", line 72, in wrapper
    return sync(self.loop, func, *args, **kwargs)
  File "/snip/lib/python3.8/site-packages/fsspec/asyn.py", line 53, in sync
    raise result[0]
  File "/snip/lib/python3.8/site-packages/fsspec/asyn.py", line 20, in _runner
    result[0] = await coro
  File "/snip/lib/python3.8/site-packages/fsspec/asyn.py", line 283, in _put
    return await _throttled_gather(
  File "/snip/lib/python3.8/site-packages/fsspec/asyn.py", line 144, in _throttled_gather
    results.extend(await asyncio.gather(*chunk, **gather_kwargs))
  File "/snip/lib/python3.8/site-packages/s3fs/core.py", line 851, in _put_file
    with open(lpath, "rb") as f0:
IsADirectoryError: [Errno 21] Is a directory: '/path/to/test_bug'

I noticed the following while investigating this:

• This issue is the same regardless of whether s3://bug exists.
• This issue is the same with any combination of trailing slashes on lpath and rpath
• The problem does not occur when copying to a subfolder within the bucket: if you replace s3://bug with s3://bug/foo then the problem goes away.
• This didn't happen on an older version (0.4.2 on python 3.6)

Environment:

• Dask version: s3fs 2021.5.0, the latest available in conda
• Python version: 3.8.10
• Operating System: Ubuntu
• Install method (conda, pip, source): conda