Improve repr string representation of SummaryCounters

[robsdedude]

Instead of for example {'nodes_created': 42, 'labels_removed': 0, '_contains_system_updates': True} the repr string representation is now SummaryCounters({'nodes-created': 42, 'labels-removed': 0, 'contains-system-updates': True}) or similar.

This is more in line with Python's recommendations:

If at all possible, this should look like a valid Python expression that could be used to recreate an object with the same value (given an appropriate environment). If this is not possible, a string of the form <...some useful description...> should be returned.

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

Further, the str representation has been adjusted from for instance {'nodes_created': 42, 'labels_removed': 0, '_contains_system_updates': True} to SummaryCounters{nodes_created: 42, contains_updates: True, contains_system_updates: True} representing all non-default attributes/properties of the object.

[MaxAake]

✅ ✏️
Just a minor spelling error in tests