Remove deprecated code from io.py

[jamesyan-git]

Remove the use of pandas.save() (deprecated since pandas 1.5). pandas.close() is now sufficient to save and close a file.
I have also updated the setup.cfg to require pandas 2.0.2 (the latest release at the time of the PR).

[jamesyan-git]

I cant seem to find any use of io.write_excel() outside of the tests though, so I'm a bit unsure how meaningfully test if it's behaving normally beyond that.

[jni]

Wow I just checked the test and it's pretty thorough, who wrote that 😂

Given the test I'm just going to merge this, thanks @jamesyan-git!