Update upper numpy revision to the stable 2.1

[sandcha]

Hi there!

I really enjoy OpenFisca, but I recently encountered an issue.

Here is what I have seen:

The current stable revision is numpy 2.1.
While today openfisca-core needs numpy 1.26.
So a major bump occurred between our used version and the stable one.

numpy 2.1 is still compatible with Python 3.10 and Python 3.11 but it has dropped the support for Python 3.9.
While openfisca-core should still be compatible with Python 3.9.

Here is what I expected to happen:

As openfisca-core syntax is connected to numpy, I expected some consistency between its supported Python and numpy revisions. And I hoped for the latest major numpy version.

Here is what actually happened:

Our currently supported numpy version is not compatible with our currently supported Python revisions.
We need to update numpy. And I understand that this will come with an update of Python; see #1287.

Context

I identify more as a:

• Analyst (I make macroscopic computations on real populations).
• Developer (I create tools that use the existing OpenFisca code).

[sandcha]

Connected to #1289 for Python update. We can hope that it will remove Python 3.9 that isn't compatible with numpy 2.1.

[bonjourmauko]

I've been testing against numpy 2.1 for a while, there is a lot of the required work already done :)