Scrapy-deltafetch

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* Drop support for Python 3.8 and lower, add support for Python 3.9 and higher.
* Add support for Scrapy 2.12.
* Use the ``REQUEST_FINGERPRINTER_CLASS`` setting introduced in Scrapy 2.7.
* Support new item types introduced in Scrapy 2.2.
* Support ``Path`` instances in the ``DELTAFETCH_DIR`` setting.

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* drop Python 2 support
* replace bsddb3 with Python's dbm for storing request fingerprints
* minor README fix
* option to disable deltafetch for some requests with deltafetch_enabled=False request meta key
* dev workflow: changed from Travis to Github Actions

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* Use python idiom to check key in dict
* Update minimum Scrapy version supported by this extension to v1.1.0

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* Log through ``logging`` module instead of (deprecated) scrapy's spider.log().
* Fix README on passing ``deltafetch_reset`` argument on the command line.

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Adds support for callbacks returning dict items.

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Fix package URL in setup.py