Inherit MutableMapping in TreeValue Class

[HansBug]

The TreeValue class need to inherit MutableMapping class for supporting all the features that dict object have.

See: https://docs.python.org/3/library/collections.abc.html#collections-abstract-base-classes

[HansBug]

• __getitem__
• __setitem__
• __delitem__
• __iter__ (Implemented, but not match with standard definition)
• __len__
• __contains__
• keys
• items (fixed)
• values (fixed)
• get (fixed: https://docs.python.org/3/library/stdtypes.html#dict.get)
• __eq__
• __ne__
• pop (fixed: https://docs.python.org/3/library/stdtypes.html#dict.pop)
• popitem
• clear
• update
• setdefault

[HansBug]

This is about dictview: https://docs.python.org/3/library/stdtypes.html#dictionary-view-objects

Maybe we need to manually build a treevalueview class for keys, values and items.