Freeze numpy version (#365)

Numpy released a new major version 2.0.0. For some reason this makes our
code crash. This was noticed in #364 where unrelated tests started
failing.

I guess the problem is due to us using an outdated pandas version.

A quick fix is implemented in this PR. The version of numpy is fixed to
the latest release before 2.0.0.

This is not the first time we freeze a dependency to an old version.
Midterm we should update our dependencies instead.

requirements.txt

@@ -17,3 +17,4 @@ SQLAlchemy<2.0.0
 sqlalchemy-stubs==0.4
 pandas==2.0.3
 pandas-stubs==2.0.2.230605
+numpy==1.26.4