Add __repr__ for introspection classes

[rytilahti]

Makes it easier to use a debugger (or print() statements) to explore the introspection data.

• Interfaces: <Interface org.freedesktop.ScreenSaver: 10 methods, 1 signals, 0 properties>
• Properties: <Property doubleClickInterval[PropertyAccess.READWRITE]: i)>
• Methods: <Method Get([<Arg interface_name[ArgDirection.IN] (s)>, <Arg property_name[ArgDirection.IN] (s)>]) -> [<Arg value[ArgDirection.OUT] (v)>]>

[acrisci]

We have one of these for a Variant. All the __repr__ functions should follow the same convention.

https://github.com/altdesktop/python-dbus-next/blob/7ba68bb40b8a21fa28b02c633c066e948bb18ce4/dbus_next/signature.py#L417-L418

https://docs.python.org/3/reference/datamodel.html#object.__repr__

Also make sure to only use this for debugging. I don't want to define an exact value for these in the library contract.