DEV: Use egg dir only for Python 2 conda package.

`preserve_egg_dir` should not be necessary in Python 3.

Author: pavoljuhas

conda-recipe/meta.yaml

@@ -9,7 +9,7 @@ source:
   git_url: ..
 
 build:
-  preserve_egg_dir: True
+  preserve_egg_dir: True        # [py2k]
 
   # If this is a new build for the same version, increment the build
   # number. If you do not include this key, it defaults to 0.