textproc/py-mkdocstrings-python-legacy: New port

This Python handler for the "mkdocstrings" package uses PyTkDocs
(= textproc/py-pytkdocs) to collect documentation from Python source
code.

Features:

• Data collection from source code: collection of the object-tree and the docstrings is done thanks to pytkdocs.

• Support for type annotations: pytkdocs collects your type annotations and mkdocstrings uses them to display parameters types or return types.

• Recursive documentation of Python objects: just use the module dotted-path as identifier, and you get the full module docs. You don't need to inject documentation for each class, function, etc.

• Support for documented attributes: attributes (variables) followed by a docstring (triple-quoted string) will be recognized by Griffe in modules, classes and even in init methods.

• Multiple docstring-styles support: common support for Google-style, Numpydoc-style, and Sphinx-style docstrings.