_pearson_r_eff_p_value fails for single point pearson values

[Zeitsperre]

Seeing this error downstream in climpred (pangeo-data/climpred#870).

xskillscore/xskillscore/core/np_deterministic.py

Line 420 in 79214ba

nan_locs = np.where(np.isnan(r))

The fix is to make use of numpy.atleast_1d():

nan_locs = np.where(np.isnan(np.atleast_1d(r)))