Prevent mixed lists

[jacqueswww]

- What I did

Removes the possibility to create mixed lists.

- How I did it

Removed MixedType and added an exception where it would usually be created.

- How to verify it

@public
def test():
    a = [1, 1.33]

should throw necessary exception.

- Description for the changelog

Mixed list support removed.

- Cute Animal Picture

[DavidKnott]

LGTM, gonna merge when I bump the version 👍

[jacqueswww]

Perfect :)