`applymap` future deprecation up to version 2.1.0

[vkehfdl1]

At pandas version v2.1.0, pd.DataFrame.map added.
So, when I use pd.DataFrame.swifter.applymap, it raises FutureDeprecation warning, and using map instead.
Change pd.applymap to pd.map for newer version.

https://pandas.pydata.org/docs/reference/api/pandas.DataFrame.map.html