Autodocsumm

This release fixes a bug that included imported members in the
autosummary table (see 32, 33) and includes the sorting bysource
(34). We also dropped the dependency on six (35).

Thanks to the input by bouweandela, felixchenier, btjanaka and
sirosen.

This release improves the implementation of autodocsumm by generating the tables using the `.. autosummary` directive. This solves multiple ambiguities within the package. We furthermore add new options:

- `:autosummary-sections:` to select specific sections to generate the autosummary tables for
- `:autosummary-no-titles` to omit section titles

Furthermore this release adds the `.. autoclasssumm::` and `.. automodulesumm::` directives to generate the autosummary tables anywhere in the documentation.

Minor fix with reference to https://github.com/Chilipp/autodocsumm/issues/24 and https://github.com/Chilipp/autodocsumm/pull/25 to not use the `Autosummary.warning` attribute.

This patch makes autodocsumm compatible with sphinx 2.4.0.

Thanks to the work by piyushk in https://github.com/Chilipp/autodocsumm/issues/19, autodocsumm now has a `:autosummary-no-nesting:` option to disable the generation of autosummary tables for the classes in a module.
As such,

.. automodule:: module
:autosummary:
:members:
:autosummary-no-nesting:

will generate an autosummary table for the specified `module`, but not for the members (i.e. classes) in the module.

See the [docs for an example](https://autodocsumm.readthedocs.io/en/latest/examples.html#generating-a-summary-table-for-the-module-without-nesting).