Constrain numpy in tests (#1203)

### Summary

Numpy released a new version which has caused our tests to fail. See
here for example in Terra:
Qiskit/qiskit#10305

### Details and comments

This PR constrains the version of numpy in the tests. We will need to
Qiskit to align with the new numpy.

Author: eggerdj

constraints.txt

@@ -0,0 +1,4 @@
+# Numpy 1.25 deprecated some behaviours that we used, and caused some
+# tests to flake. See https://github.com/Qiskit/qiskit-terra/issues/10305,
+# remove pin when resolving that.
+numpy<1.25

tox.ini

@@ -5,7 +5,7 @@ isolated_build = true
 
 [testenv]
 usedevelop = True
-install_command = pip install -U {opts} {packages}
+install_command = pip install -c{toxinidir}/constraints.txt -U {opts} {packages}
 setenv =
   VIRTUAL_ENV={envdir}
   QISKIT_SUPPRESS_PACKAGING_WARNINGS=Y