Krippendorff

* Add support for Python 3.12.
* Drop support for Python 3.8.

* Raise an error if a single coder is provided (or if all units were assigned less than 2 coders).
* Raise an error if the output `dtype` is not `np.inexact`.
* Change asserts to `ValueError`s
* Define more specific typing for the level of measurement, the distance metric callable, the NumPy arrays, and dtypes.

* Add support for Python 3.11.
* Remove support for Python 3.7.

* Fix an issue that breaks the usage of missing values (NaNs) when using a string `dtype` (thanks normangilmore for the fix!).

* Add support to Python 3.10 and drop support to Python 3.6.
* Allow specifying the return `dtype` as a parameter in the function `alpha`.

The distance metric functions now take NumPy arrays as input and output. The input arrays are broadcasted to determine the shape of the output.