The pip3 reflects that I'm using Python 3, rather than Python 2. And the -U option indicates that I'd like to upgrade my installation of mypy, if the package has been updated since I last installed it ...

For some time now, Python developers have been able to “annotate” names with type information. With Python 3.5, type hints officially became part of the language (see PEP 484).

Mypy, Pytype, Pyright, and Pyre can help you keep your type-hinted Python code bug-free. Let’s see what each of these useful tools has to offer.