Remove `list` as parent class of `TagList`

[schloerke]

Similar to #84 , I'd like to remove the parent class of list from TagList. This allows me to not have to exclude a TagList when looking for a true list item.

I don't believe I've used (but I could have!) the subset methods that a list leverages:

• append
• count
• extend
• index
• insert
• pop
• remove
• reverse
• sort

If we'd like these methods, we can implement them. Or we could ask users to leverage x.children.METHOD().