Crossmatch tests fail with scipy 1.10.0 due to `multivariate_normal_frozen` behavior change

Tests in ligo/skymap/postprocess/tests/test_crossmatch.py fail when run with the latest scipy 1.10.0 with the error

        skymap = cartesian_gaussian_to_skymap(
>           6, cartesian_gaussian.mean, cartesian_gaussian.cov)
E       AttributeError: 'multivariate_normal_frozen' object has no attribute 'cov'

This is due to an undocumented behavior change in multivariate_normal_frozen object in scipy (or better put, on the tests here relying on the rv.cov attribute being available, which is not specified in the API). Fixing this on the scipy side seems easy and might even be incorporated in a new patch version like 1.10.1, but this depends on scipy so it cannot be guaranteed.

Meanwhile, I would simply advise to not update scipy to 1.10.0 for the moment.