Update deprecated methods from Scrapy

```
Module scrapy.utils.reqser is deprecated, please use request.to_dict method and/or scrapy.utils.request.request_from_dict instead
  from scrapy.utils.reqser import request_from_dict, request_to_dict
``` 

https://github.com/scrapy-plugins/scrapy-crawlera-fetch/blob/1492753ca759a9281ed55b9ba0fb89a1a62c5e1a/crawlera_fetch/middleware.py#L18