Config-resolver

-------------

Fixed
~~~~~

* Fixed return-value of ``get_config``. It now properly returns the same return
value as config-resolver 5. New deprecation warnings have been added as well.

.. warning::
This will **BREAK** your code as ``get_config`` now returns a tuple, with
the config instance being the first element! This should never have entered
like this in the 4.x branch. Sorry about that.

* Fixed missing ``NoSectionError`` and ``NoOptionError`` imports (regression
from ``4.2.5`` via commit ``54168cd``)

-------------

Added
~~~~~

* The new "transition" function ``get_config`` now also honors the
``secure`` flag in ``lookup_options``.

-------------------

Fixes
~~~~~

* ``filename`` can now be passed as direct argument to ``get_config``
* Don't warn if the config is retrieved correctly

-------------------

Fixes
~~~~~

* Improved warning detail in deprecation messages.

-------------

Fixes
~~~~~

* Change from a module-only distrbution to a package (for PEP-561)
* Make package PEP-561 compliant
* Add transition function ``config_resolver.get_config`` for a smoother upgrade
to v5.0 in the future.
* Add deprecation warnings with details on how to change the code for a smooth
transition to v5.0

-------------

Fixes
~~~~~

* Improve code quality.
* Improve log message for invalid config version numbers.