Scrapinghub

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- add Python 3.7 support
- update msgpack dependency
- fix `iter` logic for items/requests/logs
- add `truncate` method to collections
- improve documentation

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- add an option to schedule jobs with custom environment variables
- fallback to `SHUB_JOBAUTH` environment variable if `SH_APIKEY` is not set
- provide a unified connection timeout used by both internal clients
- increase a chunk size when working with the items stats endpoint

Python 3.3 is considered unmaintained.

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- fix `iter` logic when applying single `count` param

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- add support for TZ-aware datetime objects
- better tests

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- add a client parameter to disable msgpack use
- add VCR.py json-serialized tests
- make `parent` param optional for requests.add
- improve documentation