Add type hints

[Flamefire]

This is a draft to show how it would like with type hints

It was (almost) entirely created by an AI agent (Opencode w/ Llama 3.3) to evaluate the usefulness before doing manual work.
Take as good examples:

# Clear what will be put in later
_changes: Dict[str, str] = {}
KNOWN_KEYS: List[str] = []
# Precise description of the key-values which would be harder in plain English.
# Example contents: `COMPILER_CUDA_UNIQUE_OPTS['foo'] -> (['x', 'y'], 'z')`
COMPILER_CUDA_UNIQUE_OPTS: Dict[str, Tuple[List[str], str]] = {

Currently things like this are required:

from typing import Optional, List, Union

def foo(x: List[str], y: Optional[int] = None) -> Union(str, bytes):

In Python 3.9 types are subscriptable and Union is replaced by |:

# No import

def foo(x: list[str], y: int | None = None) -> str | bytes:

There are a static analyzers/linters to verify type hints, e.g. mypy
Through that we can enforce type hints and ensure correctness on changes. Type-checking the test suite should provide good coverage of our interfaces.

cc @boegel

[jfgrimm]

If we target this for 6.0, we are planning to require python 3.9+ anyway

[Flamefire]

The earlier we do that the better. And it is not breaking anything and through the module compatible with python 3.6

When branching of EB 6.x we can remove the imports and do a (mostly) trivial search&replace. The main work is adding those, keeping them correct and consistent with new/changed code.

[jfgrimm]

that's fair