fix: remove mutable object as default for method/function values use immutable tuples as default value markers for parameters where a None value is valid. It does seem that the _marker objects are used in places where None would not be considered a valid value.

License
=======

Roundup is Free Software under the following license (from the file
COPYING.txt in the software distribution):

.. raw:: html

   <div class="file_insert">

.. include:: ../COPYING.txt

.. raw:: html

   </div>