security/py-SecretStorage: update to 2.1.2

SecretStorage 2.1.2, 2015-06-30

• Add Item.unlock() method.
• Use setuptools when it is available.
• Documentation now uses Alabaster sphinx theme.
• Other documentation fixes and improvements.

While here let it build with python3 by picking correct py-dbus port
according to python version used in the build.

PR: 204581
Submitted by: douglas@douglasthrift.net (maintainer)