(low priority) Histogram throws warning in logs in 6.0.0rc4

Bug description

When you load a dashboard with a Histogram chart, or edit one directly, this Pandas warning appears in the logs:

superset_app          | /app/superset/utils/pandas_postprocessing/histogram.py:57: SettingWithCopyWarning: 
superset_app          | A value is trying to be set on a copy of a slice from a DataFrame.
superset_app          | Try using .loc[row_indexer,col_indexer] = value instead
superset_app          | 
superset_app          | See the caveats in the documentation: https://pandas.pydata.org/pandas-docs/stable/user_guide/indexing.html#returning-a-view-versus-a-copy
superset_app          |   df[column] = to_numeric(df[column], errors="coerce")

Doesn't seem to be affecting anything in how the application operates, just something to clean up.

Screenshots/recordings

No response

Superset version

master / latest-dev

Python version

I don't know

Node version

I don't know

Browser

Not applicable

Additional context

No response

Checklist

• I have searched Superset docs and Slack and didn't find a solution to my problem.
• I have searched the GitHub issue tracker and didn't find a similar bug report.
• I have checked Superset's logs for errors and if I found a relevant Python stacktrace, I included it here as text in the "additional context" section.