elementwise comparison deprecation warnings in unit tests

[tswast]

============================================================= warnings summary =============================================================
tests/unit/test_dtypes.py::test_timearray_comparisons[left0-==-right0-expected0-date]
tests/unit/test_dtypes.py::test_timearray_comparisons[left0-==-right0-expected0-date]
tests/unit/test_dtypes.py::test_timearray_comparisons[left0-==-right0-expected0-time]
tests/unit/test_dtypes.py::test_timearray_comparisons[left0-==-right0-expected0-time]
tests/unit/test_dtypes.py::test_timearray_comparisons[left1-==-right1-expected1-date]
tests/unit/test_dtypes.py::test_timearray_comparisons[left1-==-right1-expected1-date]
tests/unit/test_dtypes.py::test_timearray_comparisons[left1-==-right1-expected1-time]
tests/unit/test_dtypes.py::test_timearray_comparisons[left1-==-right1-expected1-time]
  /Users/swast/src/github.com/googleapis/python-db-dtypes-pandas/tests/unit/test_dtypes.py:142: DeprecationWarning: elementwise comparison failed; this will raise an error in the future.
    "==": (lambda a, b: a == b),

tests/unit/test_dtypes.py::test_timearray_comparisons[left0-==-right0-expected0-date]
tests/unit/test_dtypes.py::test_timearray_comparisons[left0-==-right0-expected0-date]
tests/unit/test_dtypes.py::test_timearray_comparisons[left0-==-right0-expected0-time]
tests/unit/test_dtypes.py::test_timearray_comparisons[left0-==-right0-expected0-time]
tests/unit/test_dtypes.py::test_timearray_comparisons[left1-==-right1-expected1-date]
tests/unit/test_dtypes.py::test_timearray_comparisons[left1-==-right1-expected1-date]
tests/unit/test_dtypes.py::test_timearray_comparisons[left1-==-right1-expected1-time]
tests/unit/test_dtypes.py::test_timearray_comparisons[left1-==-right1-expected1-time]
  /Users/swast/src/github.com/googleapis/python-db-dtypes-pandas/tests/unit/test_dtypes.py:147: DeprecationWarning: elementwise comparison failed; this will raise an error in the future.
    "==": (lambda a, b: a != b),

tests/unit/test_dtypes.py: 16 warnings
  /Users/swast/src/github.com/googleapis/python-db-dtypes-pandas/.nox/unit-3-7/lib/python3.7/site-packages/numpy/core/numeric.py:2446: DeprecationWarning: elementwise comparison failed; this will raise an error in the future.
    return bool(asarray(a1 == a2).all())

tests/unit/test_dtypes.py::test_min_max_median[date]
tests/unit/test_dtypes.py::test_min_max_median[time]
  /Users/swast/src/github.com/googleapis/python-db-dtypes-pandas/.nox/unit-3-7/lib/python3.7/site-packages/numpy/lib/nanfunctions.py:1117: RuntimeWarning: Mean of empty slice
    return np.nanmean(a, axis, out=out, keepdims=keepdims)