BUG: setup.py: write version.py only after switching to Py3 build dir

This prevents importing non-2to3'ed numpy in Python 3 build, if
numpy/version.py exists for some reason but .git does not (e.g building
multiple Numpy versions from same source tree).

setup.py

@@ -149,9 +149,6 @@ def configuration(parent_package='',top_path=None):
 
 def setup_package():
 
-    # Rewrite the version file everytime
-    write_version_py()
-
     # Perform 2to3 if needed
     local_path = os.path.dirname(os.path.abspath(sys.argv[0]))
     src_path = local_path
@@ -167,11 +164,14 @@ def setup_package():
         if os.path.isfile(site_cfg):
             shutil.copy(site_cfg, src_path)
 
-    # Run build
     old_path = os.getcwd()
     os.chdir(src_path)
     sys.path.insert(0, src_path)
 
+    # Rewrite the version file everytime
+    write_version_py()
+
+    # Run build
     from numpy.distutils.core import setup
 
     try: