Backport PR #10496 on branch 5.x

[lumberbot-app]

Backport PR #10496: Define _repr_mimebundle_

Allows objects to display arbitrary mime-types by returning a mimebundle. This is getting increasingly important as custom mime-types are growing in popularity.

• mime-bundle is computed first, but other formatters are still called
• if a mime-type is present in repr-mimebundle, _repr_<mime>_ will not be called (avoids redundant calls for backward-compatible objects)

closes 10090

cc rgbkrk

Alternative design: rather than single method returning the mimebundle itself, return mime-keyed mapping to callables, e.g.:

def _repr_mime_methods_(self):
    return {
        'text/html': self._repr_html_,
    }

Another more minor alternative: rather than allowing return of data or (data, metadata), require returning the full mime-bundle with data, metadata keys:

def _repr_mimebundle(self):
    return {
        'data': {
            'application/vnd.foo+json': [1,2,3],
        },
    }