Drop from __future__ import with_statement from docs

All versions of Python that flake8 supports (2.7, 3.4+) have full
support for the with statement. There is no need to add the import.

Slightly simplifies the example be removing unnecessary boilerplate.

docs/source/plugin-development/registering-plugins.rst

@@ -25,7 +25,6 @@ like:
 
 .. code-block:: python
 
-    from __future__ import with_statement
     import setuptools
 
     requires = [