18984: [PMD] Added support for Python 2 backticks (`).

pmd-python/etc/grammar/python.jj

@@ -52,6 +52,7 @@ TOKEN : /* SEPARATORS */
 |   < COMMA: "," >
 |   < DOT: "." >
 |   < COLON: ":" >
+|   < BACKTICK: "`" >
 }
 
 
@@ -280,4 +281,4 @@ MORE : /* Strings */
 |   <"\r"> { image.setCharAt(image.length()-1, '\n'); }
 |   <~["\n","\r"]>
 |   <"\\" ~["\n","\r"]>
-}
+}