[IMP] Use explicitly relative imports in backend doc

closes #3336

Implicit relative imports have been removed in Python 3, and developers
should be encouraged to use explicitly relative imports to avoid
confusion between local and global modules

See https://www.python.org/dev/peps/pep-0328 for PEP on the subject with
reasonings and justifications

doc/howtos/backend.rst

@@ -108,7 +108,7 @@ files in the module.
 For instance, if the module has a single ``mymodule.py`` file ``__init__.py``
 might contain::
 
-    import mymodule
+    from . import mymodule
 
 Fortunately, there is a mechanism to help you set up an module. The command
 ``odoo.py`` has a subcommand :ref:`scaffold <reference/cmdline/scaffold>` to

doc/howtos/backend/exercise-creation

@@ -46,8 +46,8 @@ Index: addons/openacademy/__init__.py
 +++ addons/openacademy/__init__.py	2014-08-26 17:25:49.791783523 +0200
 @@ -0,0 +1,3 @@
 +# -*- coding: utf-8 -*-
-+import controllers
-+import models
++from . import controllers
++from . import models
 Index: addons/openacademy/controllers.py
 ===================================================================
 --- /dev/null	1970-01-01 00:00:00.000000000 +0000

doc/howtos/backend/exercise-model-inheritance

@@ -7,9 +7,9 @@ Index: addons/openacademy/__init__.py
 +++ addons/openacademy/__init__.py	2014-08-26 17:26:01.219783354 +0200
 @@ -1,3 +1,4 @@
  # -*- coding: utf-8 -*-
- import controllers
- import models
-+import partner
+ from . import controllers
+ from . import models
++from . import partner
 Index: addons/openacademy/__openerp__.py
 ===================================================================
 --- addons.orig/openacademy/__openerp__.py	2014-08-26 17:26:01.227783353 +0200

doc/howtos/backend/exercise-wizard

@@ -3,10 +3,10 @@ Index: addons/openacademy/__init__.py
 --- addons.orig/openacademy/__init__.py	2014-08-27 14:19:23.023111330 +0200
 +++ addons/openacademy/__init__.py	2014-08-27 14:22:25.043108628 +0200
 @@ -2,3 +2,4 @@
- import controllers
- import models
- import partner
-+import wizard
+ from . import controllers
+ from . import models
+ from . import partner
++from . import wizard
 Index: addons/openacademy/wizard.py
 ===================================================================
 --- /dev/null	1970-01-01 00:00:00.000000000 +0000