Python 3: use native metaclass definition syntax (#9753)
* NVDA object: metaclass syntax.
Python 2 classes can specify metaclasses via __metaclass__ attribute, whereas a metaclass keyword argument is required in Python 3. Thus use Python 3 native syntax for specifying metaclasses.
* Excel window object: metaclass syntax.
* Base object: metaclass syntax.
* Browse mode: metaclass syntax.
* Content recog: metaclass syntax.
* Acc prop server: metaclass syntax.
* Settings dialogs: metaclass syntax.
* Speech commands: metaclass syntax.
* Metaclass: remove six.with_metaclass import as Python 3 native metaclass syntax is used.
