Enable -Wunused-variable in python/PACKAGE

Summary:
This diff enables compilation warning flags for the directory in question. Further details are in [this workplace post](https://fb.workplace.com/permalink.php?story_fbid=pfbid02XaWNiCVk69r1ghfvDVpujB8Hr9Y61uDvNakxiZFa2jwiPHscVdEQwCBHrmWZSyMRl&id=100051201402394).

This is a low-risk diff. There are **no run-time effects** and the diff has already been observed to compile locally. **If the code compiles, it work; test errors are spurious.**

Reviewed By: palmje, cxxxs

Differential Revision: D66847556

fbshipit-source-id: cc1b0afa7f1a5748834025bf7eb0c9fcc02b232b

vendor/tree-sitter-python/src/parser.c

@@ -8928,7 +8928,7 @@ static bool ts_lex(TSLexer *lexer, TSStateId state) {
 
 static bool ts_lex_keywords(TSLexer *lexer, TSStateId state) {
   START_LEXER();
-  bool eof = lexer->eof(lexer);
+  lexer->eof(lexer);
   switch (state) {
     case 0:
       if (lookahead == '\t' ||