Replace applymap with map

[GuyPago]

Previously, map was used for Series class while applymap was used for the DataFrame class. Now Pandas has deprecated applymap and suggest using map for DataFrame as well.

[timkpaine]

This doesn't work on pandas<2.

[GuyPago]

Good comment. I guess requiring >=2.0 from >=0.19 is a pretty major demand, isn't it? Even though the code itself supports 2.2+.

[timkpaine]

Correct, we're not going to require >=2 so the best bet would be to check if pandas > 2/2.1 like we do here and swap the behavior you want to use.

[GuyPago]

Considering the fact that applymap is deprecated rather than removed, it feels like an overkill.
If you beg the differ, I'll integrate the change you proposed. Waiting for your response.

[timkpaine]

I made, and then reverted, this same change in #224. So I wasn't really keen to do the work just to avoid a deprecation warning.