Get Python this way?

.circleci/config.yml

@@ -458,7 +458,7 @@ jobs:
           name: "Generation version"
           command: |
             # The Nix package doesn't know how to do this part, unfortunately.
-            nix-shell -p python --run 'python setup.py update_version'
+            nix-shell .circleci/python.nix --run 'python setup.py update_version'
 
       - "run":
           name: "Build and Test"

.circleci/python.nix

@@ -0,0 +1,11 @@
+# Define a helper environment for incidental Python tasks required on CI.
+let
+  sources = import ../nix/sources.nix;
+in
+{ pkgs ? import sources."nixpkgs-21.11" { }
+}:
+pkgs.mkShell {
+  buildInputs = [
+    pkgs.python3
+  ];
+}