Integrate python version into the hash key

This prevents subtle issues when the ast changes but gast / beniget / memestra
don't get updated accordingly.

Fix #61

memestra/caching.py

@@ -154,14 +154,18 @@ class CacheKeyFactory(CacheKeyFactoryBase):
     Only the content of the module is taken into account
     '''
 
+    py_version = sys.version_info
+
     class CacheKey(object):
 
         def __init__(self, module_path, _):
 
             with open(module_path, 'rb') as fd:
                 module_content = fd.read()
-                module_hash = hashlib.sha256(module_content).hexdigest()
-                self.module_hash = module_hash
+                m = hashlib.sha256()
+                m.update(py_version)
+                m.update(module_content)
+                self.module_hash = m.hexdigest()
 
         @property
         def path(self):