generic built-in types (PEP-585)

More cool things from Python 3.9. PEP-585 introduced generic types support for the built-in types:

# before 3.9:
from typing import List, Type
lst: List[int] = [1, 2, 3]
t: Type[int] = float

# from python 3.9:
lst: list[int] = [1, 2, 3]
t: type[int] = float

So, now, from typing import will become much shorter! Hooray! The next step would be to support int & str instead of Union[int, str].

The only purpose of these types is type annotations. They don't make any runtime type checks:

list[str]({1, 2, 3})
# [1, 2, 3]

isinstance([1, 2, 3], list[str])
# TypeError: isinstance() arg 2 cannot be a parameterized generic