textproc/py-mkdocstrings-python-legacy: Update to 0.2.6

* Switch back to PyPI because since this release the sdist tarball
  contain all the required Python source files again and remove the
  no longer required workaround for "pyproject.toml".

Changelog:

https://github.com/mkdocstrings/python-legacy/releases/tag/0.2.6