fix: py lang label doesn't display in code block (#690)

Author: meteorlxy

lib/default-theme/styles/code.styl

@@ -95,7 +95,7 @@ div[class*="language-"]
       background-color $codeBgColor
 
 
-for lang in js ts html md vue css sass scss less stylus go java c sh yaml
+for lang in js ts html md vue css sass scss less stylus go java c sh yaml py
   div{'[class~="language-' + lang + '"]'}
     &:before
       content ('' + lang)