This is the kind of ad-hoc PEP proposal that could be completely avoidable if there was a generic modifier or initialization/declaration construction for declaring a variable as constant. So incredibly simple, so incredibly useful that it's hard to believe that it hasn't been accepted yet (it was proposed).
A const declaration only ensures that the variable always points to the same object, not that that object is immutable. For instance in JS:
const foo = {};
foo.bar = 1;
That depends on the language.
Something interesting to note is that Python’s ContextVar (introduced in Python 3.7) is a persistent dictionary.