add traitlets

[norweeg]

What is this Python project?

Traitlets is a pure Python library enabling:

• the enforcement of strong typing for attributes of Python objects (typed attributes are called "traits");
• dynamically calculated default values;
• automatic validation and coercion of trait attributes when attempting a change;
• registering for receiving notifications when trait values change;
• reading configuring values from files or from command line arguments - a distinct layer on top of traitlets, so you may use traitlets without the configuration machinery.

Its implementation relies on the descriptor pattern, and it is a lightweight pure-python alternative of the traits library.

What's the difference between this Python project and similar ones?

• Pure Python
• configurable from file or CLI arguments

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