Scrapinghub

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- update msgpack dependency
- drop official support for Python 3.4
- improve documentation, address a few PEP8 complaints

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- add `items.list_iter` method to iterate by chunks
- fix retrying logic for HTTP errors
- improve documentation

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- use `jobs.cancel` command name to maintain consistency
- provide basic documentation for the new feature

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- add a command to cancel multiple jobs per call
- normalize and simplify using VCR.py cassettes in tests

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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.