TYP: fix a few types
I fixed a few type issues discovered by using the pandas-stubs type tests with the pandas annotations (there are many errors).

I'm surprised that the two Excel-classes being marked as abstract wasn't detected by the CI, they are clearly meant to be instantiated, so they cannot be abstract.
@twoertwein - looks like isort is making a modification > @twoertwein - looks like isort is making a modification

All green now :) Thanks @twoertwein 