Broken due to scipy 1.5.4 being unavailable via nix.

.tests-shell.nix

@@ -2,20 +2,22 @@
 #
 # SPDX-License-Identifier: CC0-1.0
 
-{ pkgs ? import
+{ pkgs ? import 
     (builtins.fetchTarball {
       name = "nixpkgs-20.09";
       url = "https://github.com/NixOS/nixpkgs/archive/edb26126d98bc696f4f3e206583faa65d3d6e818.tar.gz";
       sha256 = "1cl4ka4kk7kh3bl78g06dhiidazf65q8miyzaxi9930d6gwyzkci";
     })
-    {
-      overlays = [
+rec {
+    unstable = import <unstable> {};
+    overlays = [
         (self: super:
 
           rec {
             python37 = super.python37.override
               {
                 packageOverrides = self: super: {
+		  scipy = unstable.python37Packages.scipy;
 
                   matplotlib = super.matplotlib.overrideAttrs
                     (oldAttrs: rec {
@@ -50,5 +52,5 @@
 }:
 pkgs.mkShell {
   buildInputs = with pkgs.python37Packages;
-    [ ipython pyfield pymagglobal cartopy matplotlib numpy scipy orthopoly ];
+    [ ipython pyfield pymagglobal cartopy matplotlib numpy scipy ];
 }