Can that really be done for Python and the vast amount of libraries? I don't get the sense that one could simply slap on a static type system on Python and get libraries typed for free.

That is unless we're talking about a gradual typing system for Python, where new Python code could be typed and integrated into untyped codebases, similar to Racket and Typed Racket.

The existing type system (see e.g. http://mypy-lang.org/) is gradual. You can mix typed and untyped code, at the cost of some safety.

You can also write type stubs for libraries that don't provide type information.