Don't import ERP5Diff to calculate the version. Avoid circular dependency building

setup.py

@@ -2,10 +2,12 @@
 # -*- coding: utf-8 -*-
 
 from setuptools import setup, find_packages
-from ERP5Diff import ERP5Diff
+import re
 
 setup(name="erp5diff",
-      version=ERP5Diff.__version__,
+      version=re.search(r'\s*__version__ = (.+)$',
+                        open('ERP5Diff.py').read(),
+                        re.MULTILINE ).group(1).strip(),
       description="XUpdate Generator for ERP5",
       author="Yoshinori OKUJI",
       author_email="yo@nexedi.com",