Use empy 3 for colcon.

Add an empy_3 package to the global scope and use it for colcon. We
don't want to simply override empy because this will affect other
packages from nixpkgs.

distros/distro-overlay.nix

@@ -6,15 +6,7 @@ let
       wxPython = pyFinal.wxPython_4_2;
 
       # ROS is not compatible with empy 4
-      empy = pyPrev.empy.overrideAttrs ({
-        pname, ...
-      }: rec {
-        version = "3.3.4";
-        src = pyFinal.fetchPypi {
-          inherit pname version;
-          hash = "sha256-c6xJeFtgFHnfTqGKfHm8EwSop8NMArlHLPEgauiPAbM=";
-        };
-      });
+      empy = pyFinal.empy_3;
     });
   };
 

pkgs/colcon/core.nix

@@ -1,5 +1,5 @@
 { lib, buildPythonApplication, buildPythonPackage, makeWrapper, fetchPypi
-, python, distlib, empy, pytest, pytest-cov, pytest-repeat
+, python, distlib, empy_3, pytest, pytest-cov, pytest-repeat
 , pytest-rerunfailures, setuptools, pytestCheckHook, flake8, flake8-blind-except
 , flake8-docstrings, flake8-import-order, pep8-naming, pylint
 }:
@@ -38,7 +38,7 @@ let
 
     propagatedBuildInputs = [
       distlib
-      empy
+      empy_3
       pytest
       pytest-cov
       pytest-repeat

pkgs/default.nix

@@ -50,6 +50,16 @@ self: super: with self.lib; let
 
       colcon-zsh = pyFinal.callPackage ./colcon/zsh.nix { };
 
+      empy_3 = pyPrev.empy.overrideAttrs ({
+        pname, ...
+      }: rec {
+        version = "3.3.4";
+        src = pyFinal.fetchPypi {
+          inherit pname version;
+          hash = "sha256-c6xJeFtgFHnfTqGKfHm8EwSop8NMArlHLPEgauiPAbM=";
+        };
+      });
+
       osrf-pycommon = pyFinal.callPackage ./osrf-pycommon {};
 
       rosdep = pyFinal.callPackage ./rosdep { };