Replace deprecated request_fingerprint with fingerprint in cache and dupefilter files

[DharmeshPandav]

request_fingerprint is deprecated in scrapy 2.7 and is replaced by fingerprint,

Using request_fingerprint results in deprecation warning in logs

https://docs.scrapy.org/en/latest/topics/request-response.html#std-setting-REQUEST_FINGERPRINTER_IMPLEMENTATION

https://github.com/scrapy-plugins/scrapy-splash/blob/6a9eb9cd35bc81bebec9719860da5c8a5d2b35bf/scrapy_splash/cache.py#L13

https://github.com/scrapy-plugins/scrapy-splash/blob/6a9eb9cd35bc81bebec9719860da5c8a5d2b35bf/scrapy_splash/dupefilter.py#L20

https://github.com/scrapy-plugins/scrapy-splash/blob/6a9eb9cd35bc81bebec9719860da5c8a5d2b35bf/scrapy_splash/init.py#L10