Update code examples for pandas 2.0 release

[scari]

I was super lucky to hear about pandas 2.0 release right before the Korean translation of the 3rd edition was about to be printed.

So, before the printing press started, I tested the example codes with pandas2 and made some necessary changes. Please take a look, and hopefully, it helps to keep the printed edition up-to-date for a bit longer, just like you wanted.

• Replaced deprecated code with up-to-date ones. eg. ExcelWriter.save()
• Modified code affected by the change in the default value of numeric_only keyword.

[wesm]

hi @scari thanks for these changes! I will update the source files and also go through accumulated errata so that O'Reilly can also do a new printing of the English version.