Unfccc-di-api

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* Use data released until 2024-01-08 when using the ZenodoReader.
* Support python 3.12.

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* Breaking: the UNFCCC restricted API access, likely you have to change your code to
use the new ZenodoReader instead.
* Add ZenodoReader which doesn't rely on API access.
* Use data released until 2023-07-18 when using the ZenodoReader.
* Build the documentation on ReadTheDocs using newer Python and Sphinx versions.

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* Support python 3.11.
* Drop support for python 3.6.

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* Fix handling of unspecified measure IDs. The DI API started returning measure IDs
without a name or description. We now call them ``unknown measure nr. {measureId}``
instead of erroring out.

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* Support python 3.10.
* Fix handling of duplicate variable IDs. **Note**: This entails changes to the public
API! In particular, UNFCCCSingleCategoryApiReader.variables now has a generic index
instead of using the ``variableId`` as index. Also, the ``query`` function now
correctly restricts queries if ``category_ids`` are provided and correctly fills
all categories with data for a multi-category variable.
* Fix pre-commit config for newer mypy type checking versions.
* Raise a more informative NoDataError (subclass of KeyError) instead of a generic
KeyError when a query result is empty.

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* Change build system.