Litresapi

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* Add new content types for podcast and podcast episode
* Drop Python <3.6 support

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* Allow to download books with different partner_id

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* Allow to download book covers with formats other than jpg

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* Fix request params for /get_the_book/ and cover download requests

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* Drop Python <3.5 support

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* Enable requests hooks for get_fresh_book
* Fix genres endpoint url
* Introduce public parse_fresh_book method to decouple fetching from parsing
* Enforce list for author, genre, sequence to simplify parsing
* Wrap annotation content to CDATA tag to avoid unparsable xml
* Better connection error handing