Option to disable Python2/comment-style type annotations

Our codebase (all Python3) has a few places where we ended up writing comments that look like # type: some description ... prior to adding Mypy. They're usually within a larger multiline comment block when we're describing an expected Dict layout, e.g.

❯ cat foo.py
# Each Foo has the following fields:
# name: name of the foo
# display_name: human name
# type: type of foo it is
# tags: list of tags that apply to the foo
❯ mypy foo.py
foo.py:4: error: invalid syntax
Found 1 error in 1 file (errors prevented further checking)

Since we're only using Python3 and "proper" type annotations, it'd be nice if we could stop Mypy from trying to parse these types of comments. I know we could change our comment style to use dashes or something else that won't anger Mypy but it's annoying that it's necessary; and honestly colons are the better delimter anyways.

Start by tracing mypy's handling of Python 2-style type comments and the configuration entry points for Python 3-only code. Reproduce the issue with the shown mypy foo.py example, then add coverage for the option; done means those comments are ignored while proper Python 3 annotations continue to be checked.