Improve setup.py and use setuptools_scm for automatic versioning.

This change means we no longer need to add commits to "bump" the version - it comes straight from the version control system.

cmmnbuild_dep_manager/__init__.py

-# -*- coding: utf-8 -*-
-
 from .cmmnbuild_dep_manager import Manager
+from ._version import version as __version__
 
-__version__ = '2.3.1'