Implement find_spec() for _SixMetaPathImporter. (#352)

six.py

@@ -71,6 +71,11 @@ def __len__(self):
             MAXSIZE = int((1 << 63) - 1)
         del X
 
+if PY34:
+    from importlib.util import spec_from_loader
+else:
+    spec_from_loader = None
+
 
 def _add_doc(func, doc):
     """Add documentation to a function."""
@@ -186,6 +191,11 @@ def find_module(self, fullname, path=None):
             return self
         return None
 
+    def find_spec(self, fullname, path, target=None):
+        if fullname in self.known_modules:
+            return spec_from_loader(fullname, self)
+        return None
+
     def __get_module(self, fullname):
         try:
             return self.known_modules[fullname]