Main build is failing with Pandas 2.1.0

[tomwhite]

From @timothymillar at https://github.com/pystatgen/sgkit/pull/1119#issuecomment-1704879496

looks like some new test failures due to upstream changes. regenie, gwas_linear_regression and bgen tests are failing with FutureWarning: unique with argument that is not not a Series, Index, ExtensionArray, or np.ndarray is deprecated and will raise in a future version. At first glance it looks like this is caused by a change in pandas 2.1.0: pandas-dev/pandas#52986