Subtraction unsupported with NumPy bool arrays

[jakirkham]

Seeing the following test failure with newer versions of NumPy. Here are the lines that this is running into.

python code/cmt/python/tests/fvbn_test.py
E..
======================================================================
ERROR: test_fvbn (__main__.Tests)
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Traceback (most recent call last):
  File "code/cmt/python/tests/fvbn_test.py", line 21, in test_fvbn
    self.assertLess(max(abs(model.input_mask() - xmask)), 1e-8)
TypeError: numpy boolean subtract, the `-` operator, is deprecated, use the bitwise_xor, the `^` operator, or the logical_xor function instead.
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Ran 3 tests in 0.014s
FAILED (errors=1)

ref: https://travis-ci.org/conda-forge/cmt-feedstock/builds/401164336