Don't supply a list with a single grouper to avoid this warning.

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#### ALL software version info
(this library, plus any other relevant software, e.g. bokeh, python, notebook, OS, browser, etc)
hvPlot Version : 0.8.0.
pandas Version : 1.5.3.
bokeh Version : 2.4.3.

Selected Jupyter core packages...
IPython : 7.31.1
ipykernel : 6.15.2
ipywidgets : 7.6.5
jupyter_client : 7.3.4
jupyter_core : 4.11.1
jupyter_server : 1.18.1
jupyterlab : 3.4.4
nbclient : 0.5.13
nbconvert : 6.4.4
nbformat : 5.5.0
notebook : 6.4.12
qtconsole : 5.2.2
traitlets : 5.1.1

#### Description of expected behavior and the observed behavior

C:\Users\shouk\anaconda3\lib\site-packages\holoviews\core\data\pandas.py:221: FutureWarning: In a future version of pandas, a length 1 tuple will be returned when iterating over a groupby with a grouper equal to a list of length 1. Don't supply a list with a single grouper to avoid this warning.
data = [(k, group_type(v, **group_kwargs)) for k, v in
C:\Users\shouk\anaconda3\lib\site-packages\holoviews\core\data\pandas.py:221: FutureWarning: In a future version of pandas, a length 1 tuple will be returned when iterating over a groupby with a grouper equal to a list of length 1. Don't supply a list with a single grouper to avoid this warning.
data = [(k, group_type(v, **group_kwargs)) for k, v in

#### Complete, minimal, self-contained example code that reproduces the issue

```
import hvplot.pandas
from bokeh.sampledata.sprint import sprint as df

boxplot = df.hvplot.box(y='Time', by='Medal', height=400, width=400, legend=False)
boxplot

```

#### Stack traceback and/or browser JavaScript console output

#### Screenshots or screencasts of the bug in action