🐛 FIX: Avoid fenced token mutation

implements: markdown-it/markdown-it@c9949dd

Author: chrisjsewell

markdown_it/renderer.py

@@ -239,7 +239,7 @@ def fence(self, tokens: Sequence[Token], idx: int, options, env) -> str:
             return highlighted + "\n"
 
         # If language exists, inject class gently, without modifying original token.
-        # May be, one day we will add .clone() for token and simplify this part, but
+        # May be, one day we will add .deepClone() for token and simplify this part, but
         # now we prefer to keep things local.
         if info:
             i = token.attrIndex("class")
@@ -248,6 +248,7 @@ def fence(self, tokens: Sequence[Token], idx: int, options, env) -> str:
             if i < 0:
                 tmpAttrs.append(["class", options.langPrefix + langName])
             else:
+                tmpAttrs[i] = tmpAttrs[i][:]
                 tmpAttrs[i][1] += " " + options.langPrefix + langName
 
             # Fake token just to render attributes