Pykechain

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* Added `Activity2.move()` function to move an Activity somewhere else under another Activity into this code base. (579 thanks to raduiordache)
* Created a framework for Property Representation, similar to the ones provided in KE-chain. Using this framework you can add representation for e.g. the SelectList, such as shown as a dropdown, checkboxes or a button. Check out the documentation on `SelectListRepresentations`. We also added `DecimalPlaces`, `SignificantDigits`, `LinkTarget` and `ButtonRepresentation`. (532 thanks to jelleboersma)
* Added `FileSizeValidator` and `FileExtensionValidator` to pykechain. Now you can use it also to create these validators for `AttachmentProperty2`-ies. Also if these validators are active on properties you can use the `Property2.is_valid()` api to check if the property conforms to these validators. With `Property2.get_reason()` you retrieve the reason for the validator being either valid or invalid. This will override the patch release of 3.0.2. 573
* Added `ImageFitValue` enum to better support the `CardWidget` and `AttachmentviewerWidget` generation and editing. 582 (thanks jelleboersma)
* Updated dependent versions for development: tox (3.14.1)

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* Ensured proper handling of filesize and fileextension validators in KE-chain. This is a temporary release for compatibility reasons. It will be replaced with fully blown Validators in the next release. (thanks to bastiaanbeijer for finding it)

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As we dropped Python 2.7 support we improve the code throughout on type hinting and type checking. We do this for
better code and code that is less prone to errors while developing python applications on top of KE-chain
with pykechain. This release improves the code in several places in this regard.

* (for developers) Additional type checking and type hinting consistencies fixed (thanks to jelleboersma)
* (for developers) added enumerations inheritance (thanks to jelleboersma)
* small fix for the `Client.user()` methods that expects a `id` keyword in the backend and got a `pk`. (thanks to jelleboersma)
* refactored the `update_dict` for bulk actions where `fvalues` can be used such as part create with properties and part update with properties (thanks to jelleboersma)

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This is a next major release of pykechain, adding support for the legacy version of the Product Information Module (PIM) in KE-chain as well as the new version PIM3.

Backward Incompatible Changes
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* this version is incompatible with Python version 2.7. It will produce a `RunTimeError` when trying to execute this in ``Python 2.7``. This is due to the fact we added Python 3 type hints to the source code to improve stability.
* When connecting to KE-chain version 2 API backends, please refer to ``pykechain version 2.7``. This versions attempts to autodetect the version of the API and switch to legacy classes and methods accordingly, but YMMV. In your requirements you can place the following line: ``pykechain <= 2.7.99`` to ensure that the latest pykechain v2 is installed.

Major differences
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* Widgets are not part of KE-chain 3. The `Activity` object does provide a `WidgetManager` to add, remove, reorder, insert and manage `WidgetSets` in general.
* There are some new widgets introduced, please refer to `the documentation <https://pykechain.readthedocs.io/en/latest/developer_api.html>`_
* We have a new `Part2`, `Property2` and `Scope2` API endpoint (``/api/v3/...``). This API is faster but asks the call to be more explicit on what fields to return initially.
* KE-chain 3 has widget level associations, and not on activity anymore. That means that parts and part models are associated per widget.
* We made over 300 commits with updates, improvements and changes in relation to pykechain v2.

Improvements
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* Added `clone_scope()` method to the `Client` and the `Scope` object. With the right permissions you can now clone a project using pykechain.
* We added 'representation' for some property types in the KE-chain 3 backend. In such way we can support alternative representations of eg. single select list as a list of buttons in the frontend, greatly improving the usability on mobile devices.
* More consistent handling of pykechain base objects throughout the code. Now you can pass in a pykechain Base subclassed object almost anywhere, where in the past you could only have passed only the UUID/id.
* We added `ref` to most pykechain models. You can find `Properties` of a `Part` based on the `id`, `name` or `ref` now. You can also search most models for its `ref`. The `ref` is a slugified value of the original name of the object in KE-chain.
* We enabled the options `check_certificates` in the `pykechain.helpers.get_project()` function and the `Client`. You can use this to disable the check for https certificates in pykechain, eg. to connect to the local HTTPS host or to a on-premise host that has a self-assigned certificate.
* We added a `DatetimeProperty` to more precisely manage the conversion of datetimes back and forth with the API.
* We added type hints on most, if not all major methods.
* We updated the documentation.
* We test pykechain version 3 against python 3.5, 3.6, 3.7, 3.8 and pypy3 - and naturally all tests pass.