Pandas deprecation warning

[0x26res]

I get this warning when running recordlinkage 13.2 with pandas 24.2

recordlinkage/base.py:288: FutureWarning: .labels was deprecated in version 0.24.0. Use .codes instead.

To avoid the warning (and the future error) the code should be updated. I don't think using codes is backward compatible though.

I think we can use this issue (and the related PR) for inspiration: https://github.com/apache/arrow/pull/3120/files

I'm happy to help/submit a PR.

[J535D165]

Thanks @arthurandres, fixed in PR #103

For reference: pandas-dev/pandas#13443

[0x26res]

Thanks!