Remove deprecated scrapy utils

[pjsier]

As of the latest Scrapy 2.10.0 release this library is running into errors because scrapy.utils.reqser has been fully removed from Scrapy after being deprecated in 2.6.0. See deprecations/removals in 2.10.0

Because this library has been stable for a while, I dropped support for Scrapy <2.6.0 since it seems safe to rely on previous versions of the library for compatibility with older versions of Scrapy and not try to maintain full backwards-compatibilty. If you'd rather add that back in I'm happy to update this.

Thanks for the review, happy to make any changes!