Fixes import statement in flaskr (#2068)

- `from flaskr.flaskr import app` in flaskr/__init__.py
  causes an import error with Python 2
- The relative import now used works for py2 and py3

docs/tutorial/packaging.rst

@@ -55,7 +55,7 @@ into this file, :file:`flaskr/__init__.py`:
 
 .. sourcecode:: python
 
-    from flaskr import app
+    from .flaskr import app
 
 This import statement brings the application instance into the top-level 
 of the application package.  When it is time to run the application, the 

examples/flaskr/flaskr/__init__.py

@@ -1 +1 @@
-from flaskr import app
+from .flaskr import app

examples/flaskr/setup.cfg

@@ -1,2 +1,2 @@
-[aliases]
+[tool:pytest]
 test=pytest