Fix: #445 - add __len__ to listbox with validation if body Sized

[penguinolog]

• If body is not Sized - __len__ call should raise AttributeError
• Downside: since body can be replaced, __len__ is always set ant ListBox by itself will pass Sized instance check
• Relax SimpleListWalker constructor rules: any Iterable is allowed as source due to list is used internally

Checklist

• I've ensured that similar functionality has not already been implemented
• I've ensured that similar functionality has not earlier been proposed and declined
• I've branched off the master or python-dual-support branch
• I've merged fresh upstream into my branch recently
• I've ran tox successfully in local environment
• I've included docstrings and/or documentation and/or examples for my code (if this is a new feature)

Fix #445