Use modern Python type annotation syntax

[scotts]

We still use the old syntax and types. For example, we say something like:

def foo(x: Optional[List[Union[int, float]]]):

While the newer way would be:

def foo(x: list[int | float] | None):

Using list versus List is supported since Python 3.9, and the better optional and union syntax since 3.10. Our minimum has been 3.10 since TorchCodec v0.8. See: https://docs.python.org/3/library/typing.html

[NicolasHug]

There are tools to automatically update a Python codebase to use the Python features of a given version. We don't have to do all this manually, I forgot the name of the lib though 🤔