Add type hints

[Daverball]

These type hints I had to create from scratch, but were by far the most straightforward.

It is however worth noting that by adding the type hints, it came to light, that scan currently does not handle MetaPathFinder importers, so we maybe should open an issue for that and try to add support for them in the future.

I also noticed that there's some strange logic when it comes to packages vs. plain modules. Since walk_packages always imports packages, we technically don't need to import them again in scan (even though going through __import__ guarantees that the actual import only happens once). The other thing that's strange about that is that we use a import_module helper function in scan to skip loading some modules, but we don't do that in walk_packages, so the check only really works on plain modules, not packages. This might also be worth investigating and opening a ticket for.

I also decided to get rid of the workarounds for old finders that only have a find_module, but no find_spec method, since those have been deprecated since 3.4 and fully unsupported since 3.9, so those finders would not work with 3.10 anyways and importing the module would fail.

/cc @henri-hulski

[henri-hulski]

If you want to modernize importscan,feel free to go. I wasn't digging into it to much and I don't remember that there was any work done beside modernizing the workflow.
Other than Morepath or Reg.
So I think it can use some review.