Px pandas2

[alexcjohnson]

Fixes #4179
Fixes #4065

Pandas v2.0 removed df.append, need to change to pd.concat. Note that pd.concat has been available since at least v0.13 so there should be no issue with older versions requiring that we use df.append.

Tested by having the px Percy tests generate the same HTML files with both pandas==1.1.5 (the oldest version with a Py3.9 wheel) and pandas==2.0.1 (latest) and check that the HTML is identical before shipping it off to Percy.

• I have read through the contributing notes and understand the structure of the package. In particular, if my PR modifies code of plotly.graph_objects, my modifications concern the codegen files and not generated files.
• I have added tests (if submitting a new feature or correcting a bug) or
  modified existing tests.
• I have added a CHANGELOG entry if fixing/changing/adding anything substantial.

[alexcjohnson]

OK that test was a little naive, if nothing else because the figure gets a UUID as its div ID. I'll fix that in the morning and add a changelog

[LiamConnors]

Looks good to me 💃