Remove depreciated declare_namespace()` (#177)

This fix removes declare_namespace() which is no longer needed.
it used old style namespaces which has been using `pkg_resources` module
which is part of the `setuptools` and which is as well depreceted now.

Signed-off-by: Tomasz Kłoczko <kloczek@github.com>

Author: kloczek

sphinx_tabs/__init__.py

@@ -1,3 +1 @@
 __version__ = "3.4.1"
-
-__import__("pkg_resources").declare_namespace(__name__)