In python 3.6, a loose statement that looks like "key: value", but outside of a dictionary context, is valid syntax. What is this syntax used for?

Question

I'm guessing this is documented somewhere and has been discussed before, and I will close this question if someone finds a duplicate.

In python 3.6 I can enter the line

foo: "bar"

or

x: 1.2345

and this appears to be valid syntax. That is, any set of characters formatted like a variable name followed by a colon then some object does not raise an error. I've been using python for quite a while so this was really surprising to me. What is the interpreter seeing here? What is this for?

I came across this when reading about python3.7 and the new @dataclass decorator; I noticed the examples use the syntax

@dataclass
class Color:
    lightness: float = 0.5
    hue: float
    saturation: float

and assumed this loose key: value syntax was new. Evidently not.