Fix recursion in pytest.approx() with arrays in numpy<1.13

[nicoddemus]

Fix #3748

cc @ironiccode @tigarmo

I tested this patch manually using this example from @tigarmo using numpy 1.11:

def testApprox():
    from pytest import approx
    import numpy as np

    a = np.array([1, 2, 3])
    assert a == approx([1, 2, 3])

I did not add tests for this because it requires numpy<1.13. I thought it overkill to add another CI environment because of this.

[RonnyPfannschmidt]

blanked approval to support the merge,
the related parties are capable and i'm happy to see this merged once verified

[coveralls]

Coverage increased (+0.05%) to 92.547% when pulling b825530 on nicoddemus:numpy-recursion-error into 4588130 on pytest-dev:master.

[kalekundert]

This was a pretty weird bug, so I thought it would be prudent to put a comment in the code indicating why the comparison has to be done in a certain way.