* Add a descriptor to have a class level constant
This descriptor helps intercept places where we set a value on instances.
It does not really behave like "const" in C-like languages, but is the
simplest implementation that might still be useful.
* Add a descriptor to deprecate properties/attributes of an object
This descriptor is used as a base class. Derived classes may implement a
factory to return an adaptor to the attribute being deprecated. The
descriptor can either warn, or raise an error, when usage of the deprecated
attribute is intercepted.
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Co-authored-by: Harmen Stoppels <me@harmenstoppels.nl>
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