PostgresRangePartitioningStrategy should extend PostgresPartitioningStrategy or type hints should be fixed

[danwyry]

according to type hints in PostgresPartitioningConfig, the strategy parameter expects a subtype of PostgresPartitioningStrategy, but PostgresRangePartitioningStrategy is not, so linters detect this inconsistency in the code an hightlights it as an error.
not a real issue, but still easy and worth the fix.