from_clipboard doesn't work by default when copying string cells from excel

need to do pd.read_clipboard(sep='\t') and then things work fine.
Since it's for interactive use, it should be ok to change the default
without breaking existing code.

we did the same to to_clipboard() in #5070, I missed the fact that
read_clipboard only works out of the box for pure numerical data right now.

Why don't users report this stuff I don't know.