[DEPENDENCY HOLD] Almost fix #13539: use Sizzle.selectors.attrHandle instead of overriding Sizzle.attr

Don't commit until the Sizzle [dependency](https://github.com/jquery/sizzle/pull/200) lands, but the numbers speak for theirselves:
```
   raw     gz Sizes                                                             
270935  80384 dist/jquery.js                                                    
 93054  32735 dist/jquery.min.js                                                

   raw     gz Compared to 1.9-stable @ e3a802cbf7d66d88e3659ad344bce86b99d029be 
   -16   -272 dist/jquery.js                                                    
   -47   -106 dist/jquery.min.js
```

Almost half of the savings are independent of Sizzle changes, but 64 bytes off of jquery.min.js is still nothing to sneeze at. I'll make an analog for 2.x after this one is cleared.