bpo-30629: Remove second call of str.lower() in html.parser.parse_endtag. (#2099)

elem is the result of .lower() 6 lines above the handle_endtag call.
Patch by Motoki Naruse

Lib/html/parser.py

@@ -418,7 +418,7 @@ class HTMLParser(_markupbase.ParserBase):
                 self.handle_data(rawdata[i:gtpos])
                 return gtpos
 
-        self.handle_endtag(elem.lower())
+        self.handle_endtag(elem)
         self.clear_cdata_mode()
         return gtpos