MAINT: make assert_allclose behavior on nans match pre 1.12

[person142]

Before:

>>> import numpy as np
>>> np.__version__
'1.11.2'
>>> np.testing.assert_array_equal(np.nan, np.nan)
>>> np.testing.assert_array_almost_equal(np.nan, np.nan)
>>> np.testing.assert_array_less(np.nan, np.nan)
>>> np.testing.assert_allclose(np.nan, np.nan)

Now:

>>> np.__version__
'1.12.0.dev0+6da5f00'
>>> np.testing.assert_array_equal(np.nan, np.nan)
>>> np.testing.assert_array_almost_equal(np.nan, np.nan)
>>> np.testing.assert_array_less(np.nan, np.nan)
>>> np.testing.assert_allclose(np.nan, np.nan)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/Josh/Projects/numpy-dev/python3/numpy/numpy/testing/utils.py", line 1388, in assert_allclose
    verbose=verbose, header=header, equal_nan=equal_nan)
  File "/Users/Josh/Projects/numpy-dev/python3/numpy/numpy/testing/utils.py", line 773, in assert_array_compare
    raise AssertionError(msg)
AssertionError:
Not equal to tolerance rtol=1e-07, atol=0

(mismatch 100.0%)
 x: array(nan)
 y: array(nan)

Admittedly np.assert_allclose(np.nan, np.nan) raising seems more natural, but the changes make some old tests fail. For context see:

scipy/scipy#6696
scipy/scipy#6697

[rgommers]

Admittedly np.assert_allclose(np.nan, np.nan) raising seems more natural

Only from a "this is how floating point numbers work" point of view. If you compare a test result with expected values, then the nans in the given expected values are there on purpose and should compare equal for testing purposes.

[charris]

Thanks @person142 .