Compat with pandas 0.24 rename of MultiIndex labels -> codes

[asfimport]

Pandas deprecated the MultiIndex.labels in favor of MultiIndex.codes (pandas-dev/pandas#23752 In the pandas parquet/feather tests, we are now seeing warnings about this (and I assume there will be warnings as well in pyarrow tests if running on pandas master).

Reporter: Joris Van den Bossche / @jorisvandenbossche
Assignee: Joris Van den Bossche / @jorisvandenbossche

PRs and other links:

• GitHub Pull Request #3120

_{Note: This issue was originally created as ARROW-3953. Please see the migration documentation for further details.}

[asfimport]

Krisztian Szucs / @kszucs:
Issue resolved by pull request 3120
#3120