Fixtures loader improvements

[greg0ire]

Please add a description, and use separate commits with good messages for each thing you do. You can learn more about this in the contributing guide.

[greg0ire]

When I say "use separate commits with good messages for each thing you do" I mean this:

• add return type hints
• remove extension in filename
• configure recursive directory iterator

I don't mean this:

• improve stuff
• revert type hints
• fix cs
• …
• more fix cs

We don't care that you need to fix cs, we want explanations for very different issues you happen to fix in the same files.