Using isinstance instead type

This patch uses isinstance, like Thomas pointed in their last commit.

offlineimap/folder/IMAP.py

@@ -482,7 +482,7 @@ class IMAPFolder(BaseFolder):
                              item[1], flags=re.IGNORECASE):
                     found = item[0]
             elif found is not None:
-                if type(item) == type(b""):
+                if isinstance(item, bytes):
                     item = item.decode('utf-8')
                     uid = re.search("UID\s+(\d+)", item, flags=re.IGNORECASE)
                     if uid: