Remove Category xdist serialization/deserialization

[blake-r]

This PR removes the custom serialization (save) and deserialization (load) methods for the Category enum, as well as the StrEnum base class for Category. Instead, Category is redefined using a metaclass (CategoryMeta) to provide iteration capabilities while being fundamentally a plain string type.

The primary motivation is to simplify the data types used for communication between master and worker processes in xdist. The Category type is now represented as a simple str in type hints (CategoryT = str in typing.pyi), removing the need for explicit serialization steps when sending CategoryMeasurementsT dictionaries over the wire. The dump_measurements function now simply passes the dictionary through, and load_measurements uses the string keys directly as dictionary keys for the destination.

This change aims to make the xdist communication more robust and efficient by relying on native, serializable types.