In your first example, where you inherit from wx.ToolBar and abc.ABCMeta, you don't want AppToolbar to be a subclass of abc.ABCMeta, you want AppToolbar to be an instance of it. Try this:
class AppToolbar(wx.ToolBar, metaclass=abc.ABCMeta):
# ... as above, but with the following added
@abc.abstractmethod
def _PopulateToolbar():
pass
Though looking at this a bit closer, it seems that you can't define a subclass of wx.Toolbar with abc.ABCMeta as its metaclass, as wx.Toolbar is an instance of a metaclass other than bultins.type. You can, however, get abstract-like behavior out of AppToolbar._PopulateToolbar:
class AppToolbar(wx.ToolBar):
def _PopulateToolbar():
''' This is an abstract method; subclasses must override it. '''
raise NotImplementedError('Abstract method "_PopulateToolbar" must be overridden before it can be called.')