Keep imports alphabetically sorted and their order homogeneous across
Python source files.
The isort project has more feature and is more active than the
flake8-import-order plugin.
Most issues caught were simply import ordering from the same module.
Where imports were purposefully placed out of order, tag with
isort:skip.
This allows `autodoc-skip-member` handlers to determine whether a member
not included in `__all__` attribute of the module should be documented or
not.
autodoc raises a parsing error if some extension generates a docstring
not having blank lines at the tail. This appends a blank line if
generated one does not contain it.
