mypy error on progress bar usage

xxx.py:
```python
import click

with click.progressbar([1, 2, 3]) as bar:
    for x in bar:
        pass
```

mypy error:
```
$ mypy xxx.py 
xxx.py:4: error: "Iterable[<nothing>]" has no attribute "__next__"  [attr-defined]
Found 1 error in 1 file (checked 1 source file)
```

Environment:

- Python version: 3.9.5
- Click version: 8.0.0
- mypy version: 0.812
