Fix missing namespace declaration on ext level in __init__.py 

Avoid:

Getting distribution for 'zopyx.txng3.ext'.
error: Setup script exited with error: Namespace package problem: zopyx.txng3.ext is a namespace package, but its
__init__.py does not call declare_namespace()! Please fix it.
(See the setuptools manual under "Namespace Packages" for details.)
"
An error occurred when trying to install zopyx.txng3.ext 3.3.5. Look above this message for any errors that were output by easy_install.
While:
  Updating instance.
  Getting distribution for 'zopyx.txng3.ext'.
Error: Couldn't install: zopyx.txng3.ext 3.3.5