shared: fix signature test under Python 3

slapos/recipe/downloadunpacked.py

@@ -243,7 +243,7 @@ class Signature:
   def update(self, key, value):
     self.item_list.append(('%r: %r' % (key, value)).encode())
 
-  def hexdigest(self):
+  def hexdigest(self): # -> str
     m = md5()
     for item in self.item_list:
       m.update(item)
@@ -253,8 +253,9 @@ class Signature:
     return b'\n'.join(self.item_list)
 
   def test(self, folder_path):
+    assert type(folder_path) is str, folder_path
     digest = self.hexdigest()
-    if os.path.basename(folder_path).encode('utf-8') == digest:
+    if os.path.basename(folder_path) == digest:
       target_path = os.path.join(folder_path, self.filename)
       if os.path.exists(target_path) and open(target_path, 'rb').read() == self.dumps():
         return True