Add override for isort to cover missing dependency on setuptools

Without this, the wrapped isort binary cannot import setuptools.

Upstream, this will eventually be fixed when the code containing this
line gets released:

3e09facb23/isort/main.py (L19)

overrides.nix

@@ -171,6 +171,13 @@ self: super:
       }
     );
 
+  isort = super.isort.overridePythonAttrs
+    (
+      old: {
+        propagatedBuildInputs = old.propagatedBuildInputs ++ [ pkgs.setuptools ];
+      }
+    );
+
   jupyter = super.jupyter.overridePythonAttrs
     (
       old: rec {