Remove invalid argument unpacking in normalize_cats

[jhaddixcodes]

I believe this line (and the one a few lines below it) in normalize_cats will almost always raise a TypeError as append can only take one argument, but unpacking a list as arguments could yield 0 or 2+ arguments:

final_cats.append(*to_be_pushed_cats)

This occurs any time the to_be_pushed_cats or to_be_pushed_subcats lists have 0 or more than 1 member, as this means the unpacking results in an invalid number of arguments for append.

The standard way of doing this is to instead use the extend method of list which handles iterables correctly:

final_cats.extend(to_be_pushed_cats)