tweak the fit of the parts

lib/feather_case_wizard/toolchain.rb

@@ -4,18 +4,20 @@ module FeatherCaseWizard
   class ToolError < StandardError; end
 
   class Toolchain
-    attr_reader :openscad, :blender
+    attr_reader :openscad, :blender, :python
 
     def initialize(env: ENV)
       @env = env
       @openscad = discover("OPENSCAD", "openscad")
       @blender = discover("BLENDER", "blender")
+      @python = discover("PYTHON", "python3")
     end
 
     def check!
       missing = []
       missing << "OpenSCAD (`sudo apt-get install openscad` or set OPENSCAD)" unless openscad
       missing << "Blender (`sudo apt-get install blender` or set BLENDER)" unless blender
+      missing << "Python 3 (`sudo apt-get install python3` or set PYTHON)" unless python
       return self if missing.empty?
 
       raise ToolError, "Missing required tools:\n  - #{missing.join("\n  - ")}"