Krippendorff

* 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.

* Add support for Python 3.9.
* Remove support for Python 3.5.
* Add typing annotations.

* Remove pypandoc setup dependency.
* Add missing numpy install dependency.

Remove support to Python 3.4 and declare explicit support to Python 3.8.

* Fix 'ordinal' metric, which was giving a slightly different value for alpha.
* Remove support to Python 3.3.