Pykechain

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* :bug: removed use of the old API usage of `descendants`. (741)
* :+1: We will now raise the correct errors when retrieving an `active banner`. (741)
* :+1: We now use actual depth-first sorting of children. (741)
* :+1: For testing and development: Less hard-coded and centralized teardowns, using `assertEqual(expected, received)` format. (741)
* :+1: dependent versions for development: Sphinx (3.0.2), nbsphinx (0.6.1)

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* :+1: Added `current_user()` method on the Client class to return the `User` object of the user connected to KE-chain.
* :+1: Added various arguments to the `create_notification()` method.
* :+1: Added `edit()` method to the Notification class.
* :bug: Removed old API usage of `descendants` from `copy()` of the Part class.
* :bug: Raising correct errors whenever no (or multiple) active banner exist in the `active_banner()` method.

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* :bug: We found out that the regex to validate the email addresses was incorrectly defined. We added additional tests to fix this. (thanks to jberends for the omission and the fix)

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This was never released due to some inappropriate tagged.

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This minor releases adds two new concepts (:star: :star:) that also exist in the KE-chain backend. We added a `Banner` concept that allows powerusers
to set an announcement banner which is displayed (and can be dismissed) in KE-chain for all logged-in users within a certain timeperiod. We also added the concept of a `Notification` in KE-chain, where email or in-app notification can be displayed suchs as sharing a link of
a task to another user, or sharing a PDF of a task to another user or and external email address.

* :star: Implemented `Banner` model and methods to create, delete and retrieve them. 725 (Thanks to jelleboersma for the implementation)
* :star: Added the concept of a `Notification` including the possibility to retrieve, create(send) and delete `Notifications`. 467 (thanks to raduiordache for the implementation)
* :star: Added the possibility to share a link or PDF of an activity using the `Activity2.share_pdf()` or `Activity2.share_link()`. 467 (thanks to raduiordache for the implementation)
* :bug: When adding a new `Activity2` to a parent, it will now cache its childeren in case the parent also have the children cached. In essence it updates the `_cached_children`. 722 (thanks to jelleboersma)
* :+1: dependent versions for development: pytest (5.4.1), mypy (0.770), tox (3.14.6)

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* :star: implemented retry on connection errors for the `Client`. This will ensure that if the client connection to the server has been dropped, the client will retry the request again with an exponentional backoff not to overload. 714
* :+1: dependent versions for development: sphinx (2.4.4)