Bound Diffrax version (qiskit-community#226)

* Bounding diffrax and equinox versions. The latest versions require
the latest version of JAX, but due to an unresolved bug in JAX,
Dynamics is only compatible with jax<=0.4.6. This commit also
adds a release note stating exactly what versions of these packages
will work with the latest version of dynamics.

releasenotes/notes/diffrax-bound-0bd80c01b7f4b48f.yaml

@@ -0,0 +1,5 @@
+---
+issues:
+  - |
+    Due to a bug in JAX, Dynamics can only be used with jax<=0.4.6. As they depend on newer versions
+    of JAX, Dynamics is also now only compatible with diffrax<=0.3.1 and equinox<=0.10.3.

tox.ini

@@ -17,14 +17,16 @@ deps =
     -r{toxinidir}/requirements-dev.txt
     jax<=0.4.6
     jaxlib<=0.4.6
-    diffrax
+    equinox<=0.10.3
+    diffrax<=0.3.1
 
 [testenv:lint]
 deps =
     -r{toxinidir}/requirements-dev.txt
     jax<=0.4.6
     jaxlib<=0.4.6
-    diffrax
+    equinox<=0.10.3
+    diffrax<=0.3.1
 commands =
   black --check {posargs} qiskit_dynamics test
   pylint -rn -j 0 --rcfile={toxinidir}/.pylintrc qiskit_dynamics/ test/