Failing tests in tests/test_core.py

[andersy005]

Per a recent CircleCI report, the following tests have issues that require further investigation

• tests/test_core.py::test_mon_climatology[cesm_cice_daily-False-variables4-time]
• tests/test_core.py::test_mon_climatology_drop_time_bounds[tiny-False-variables1-time-bounds]
• tests/test_core.py::test_mon_climatology_drop_time_bounds[cmip5_pr_amon_csiro-False-variables2-time-bounds]
  tests/test_core.py::test_mon_climatology_drop_time_bounds[cesm_cice_daily-False-variables4-time-bounds]