Pykechain

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* Added bulk widget creation and editing of widgets. 617 (thanks to jelleboersma)
* Added methods to retrieve pykechain objects from the server via 'ref'. Including services. 608. (thanks to raduiordache)
* Added `edit` and `delete` methods to `Team` class and fixed some bugs relating to `Team` creation. 620 (thanks to jelleboersma)
* Added additional inputs to create an `Activity`: `status`, `start_date`, `due_date`, `description`, `classification`. 615 (thanks to jelleboersma)
* Bugfix: `Part.scope()` retrieves the part's scope regardless of its status. (thanks to jelleboersma)
* Improved `Client` exception messages when retrieving singular objects, e.g. `Client.scope()` (thanks to jelleboersma)
* Updated dependent versions for development: mypy (0.750), tox (3.14.2), sphinx (2.2.2), coveralls (1.9.2 :vulcan_salute:️)

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This is the black friday edition of pykechain.

* Changed the default upload of a sim script to use python 3.6 when executed on KE-chain as a script.
* Added tests for retrieving objects by `ref`. (608 - thanks to jelleboersma for the find and raduiordache for the PR)
* Updated the implementation of the `add_scope_widget()` method to support filters. (thanks to jelleboersma)
* Updated dependent versions for development: twine (3.1.1), pytest (5.3.1)

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* Updated the implementation of the `Scope.members()` method when dealing with `is_leadmember` and `is_manager` filters. (thanks to jelleboersma)
* Updated CI tests to use Github Actions.

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* Fixed the bulk editing and creating of parts (`Part2`) which have attachments in the list of properties. In the background we now separate the upload of attachments from the update of the properties. 590 (Thanks to jelleboersma)
* Fixed a bug with timezones.
* Updated dependent versions for development: nbsphinx (0.5.0), pytest (5.3.0), jsonschema (3.2.0), twine (3.0.0), pyopenssl (19.1.0).

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* small fix for backwards compatibility of `CardWidgetLinkTarget` enum.

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* This version is never released to the public