Replace raw dict by CategorizedPaths object (NamedTuple(-like))

[Flamefire]

The current dict is pretty opaque and makes spelling mistakes easy.
Avoid by using a typed, named tuple.

There should be no other entries as this is mostly an internal class but the backwards-compat path allows it for now.

In EB 6.0 this can be a pure NamedTuple or a dataclass

[boegel]

In general, I like this, it indeed leaves less room for mistakes.

However, we're changing the function signature and or return type of several functions that are part of our API here:

• categorize_files_by_type now returns a CategorizedPaths instance, instead of a dict;
• new_branch_github, new_pr_from_branch, new_pr, det_pr_target_repo, update_branch, update_pr now take a CategorizedPaths instance as a value, rather than the dict they used to take before;

Even if we wait with merging this for EasyBuild 6.0, we should still deprecate the old function signatures.

This could be done by:

• adding an option to categorize_files_by_type to determine the type of return value (dict by default, opt-in to CategorizedPaths instance);
• auto-convert a dict value passed to new_pr & co to a CategorizedPaths instance internally, if needed (so accept both dict and CategorizedPaths instance as input value);

I agree that it's unlikely that people would be using one of these functions, but if anyone does (for example in a Python script that leverages EasyBuild framework as a library, see also here), then we would be breaking their script.

Backwards compatibility is a strong focus point of EasyBuild, and I very much want to keep it like that...

[boegel]

Why not use NamedTuple already? It probably requires a newer Python version as minimal version?
If so, mention that in the comment (and put it above the class line while you're at it)

[Flamefire]

We need the dict-interface. Without that the NamedTuple (or dataclass in 3.9) from 4a79421 are enough:

class CategorizedPaths(NamedTuple):
    easyconfigs: List[str]
    easyconfigs: List[str]
    files_to_delete: List[str]
    patch_files: List[str]
    py_files: List[str]

Enhanced the comment and added another one below

[Flamefire]

This could be done by:

* adding an option to `categorize_files_by_type` to determine the type of return value (`dict` by default, opt-in to `CategorizedPaths` instance);

* auto-convert a `dict` value passed to `new_pr` & co to a `CategorizedPaths` instance internally, if needed (so accept both `dict` and `CategorizedPaths` instance as input value);

Is that an either-or or both?
I would have said the first is enough but we can be extra save and do both. Now commit for the 2nd point added