Don't make assumptions in List.append() and extend()

[dloscutoff]

Comment in List.append(): "This assumes that item is an instance of a Pip type--unpredictable behavior may follow if it's not." Instead of assuming, it would be pretty simple to check and raise TypeError when appropriate, which would be a lot better than unpredictable behavior. Same goes for List.extend(), though the check will be a bit more complicated.