text: Title: Deprecate unused scrapy.utils code
  Body: As far as I can see the following functions and classes from `scrapy.utils` are not used anywhere (usually because the code using them is gone):
  
  * `scrapy.utils.python.flatten()`
  * `scrapy.utils.python.iflatten()`
  * `scrapy.utils.python.equal_attributes()`
  * `scrapy.utils.request.request_authenticate()`
  * `scrapy.utils.serialize.ScrapyJSONDecoder`
  * `scrapy.utils.test.assert_samelines()`
  
  `flatten()` and `iflatten()` are also present in Parsel where they are used but I wouldn't recommend switching to those (and they are easy to reimplement anyway).