Python: flatten-dict package

[jpmckinney]

Check whether https://pypi.org/project/flatten-dict/ has similar performance to OCDS Merge's routines, and whether we can use it to eliminate some flattening logic in ocds-merge and lib-cove-ocds (flatten_dict).

Note: There might also be similar logic in the external libraries lib-cove (OpenDataServices/lib-cove#60) and sphinxcontrib-opendataservices (flatten_dict).

[jpmckinney]

Long-term we want to use Rust for OCDS Merge https://github.com/open-contracting/ocds-merge-rs

flatten_dict in lib-cove-ocds is only used for an additional check, which is now skipped in Kingfisher Process (as the check is performed by Pelican instead).

sphinxcontrib-opendataservices doesn't have performance needs, so this issue would have only reduced code duplication. We don't maintain that library though.

lib-cove has other more important performance bottlenecks (OpenDataServices/lib-cove#65) that wouldn't benefit from this issue.