CI: failing builds on master

[jreback]

https://travis-ci.org/pandas-dev/pandas/builds/599134469

looks like some issues on 3.8, 3.6 allowed failure builds (i know has been failing a while)

should address these before merging other changes.

cc @pandas-dev/pandas-core

[jbrockmendel]

Looks like a new difference between numpy 1.17 vs 1.18:

s = pd.Series(
            pd.to_datetime(["NaT", "2000-1-2", "NaT", "2000-1-1", "NaT", "2000-1-3"])
        )

>>> np.minimum.accumulate(s.values)  # numpy 1.17
array([                          'NaT', '2000-01-02T00:00:00.000000000',
       '2000-01-02T00:00:00.000000000', '2000-01-01T00:00:00.000000000',
       '2000-01-01T00:00:00.000000000', '2000-01-01T00:00:00.000000000'],
      dtype='datetime64[ns]')

>>> np.accumulate.minimum(s.values)  # numpy 1.18
array(['NaT', 'NaT', 'NaT', 'NaT', 'NaT', 'NaT'], dtype='datetime64[ns]')

[jbrockmendel]

numpy/numpy#14717

[jbrockmendel]

closed by #29057