Robot Framework 6.1 and Pandas 2.0

This Release fixes compatibility to Robot Framework 6.1 (fixes #88 )

And Pandas 2.0 introduced that by default the string "None" in an Excel cell is interpreted as a None value, which is NaN in pandas.
This is fixed as well.

I made some more refactoring related to Paths and if you find any issue, with this version, please report asap.

Thanks and Cheers
René

Full Changelog: 1.7.0...1.8.0