rtd: force devito version (#619)

Author: mrava87

requirements-doc.txt

@@ -1,5 +1,7 @@
 # Currently we force rdt to use numpy<2.0.0 to build the documentation
-# since the dtcwt and spgl1 are not yet compatible with numpy=2.0.0
+# since the dtcwt is not yet compatible with numpy=2.0.0. For the
+# same reason, we force devito==4.8.7 as later versions of devito
+# require numpy>=2.0.0
 numpy>=1.21.0,<2.0.0
 scipy>=1.11.0
 jax
@@ -11,7 +13,7 @@ PyWavelets
 spgl1
 scikit-fmm
 sympy
-devito
+devito==4.8.6
 dtcwt
 matplotlib
 ipython