Require at least python-stdnum >= 1.8

It is the minimal version to use get_cc_module from stdnum module.

issue8646
review264191002

setup.py

@@ -45,7 +45,7 @@ if minor_version % 2:
         'hg+http://hg.tryton.org/modules/%s#egg=%s-%s' % (
             name[8:], name, version))
 
-requires = ['python-sql >= 0.4', 'python-stdnum']
+requires = ['python-sql >= 0.4', 'python-stdnum >= 1.8']
 for dep in info.get('depends', []):
     if not re.match(r'(ir|res)(\W|$)', dep):
         requires.append(get_require_version('trytond_%s' % dep))