Use metaclass repr

Checks for metaclass's `__repr__` in `_type_pprint`
    
`RepresentationPrinter` has a type pprinter for 'type' subclasses. It will thus be called to print a class object (instance of 'type'), avoiding the `__repr__` of the metaclass.
    
This patch checks for an overridden `__repr__` before using the custom type-representation logic.

Fixes #6709

I made this patch against 2.3.0 but I could rebase this against master if you want.