cloudpickle v1.5.0 breaks testing

[matthewfeickert]

Description

With the release of cloudpickle v1.5.0 on 2020-07-01 the CI is broken in testing as the following error is raised

ImportError while loading conftest '/home/runner/work/pyhf/pyhf/tests/conftest.py'.
tests/conftest.py:83: in <module>
    (pyhf.tensor.tensorflow_backend(), None),
src/pyhf/tensor/__init__.py:44: in __getattr__
    e,
E   pyhf.exceptions.ImportBackendError: ('There was a problem importing TensorFlow. The tensorflow backend cannot be used.', ImportError("cannot import name 'CloudPickler' from 'cloudpickle.cloudpickle' (/opt/hostedtoolcache/Python/3.7.7/x64/lib/python3.7/site-packages/cloudpickle/cloudpickle.py)"))
##[error]Process completed with exit code 4.

cloudpickle is a required dependency of TensorFlow Probability and in TFP v0.10.0 it is set to cloudpickle >= 1.2.2.

This has been reported in:

• TensorFlow Probability Issue 991
• cloudpickle Issue 390

Expected Behavior

For no error to be raised

Actual Behavior

c.f. above

Steps to Reproduce

This was found in CI, but the minimal test case is just to install TensorFlow and TensorFlow Probability and then try to import TFP:

$ python -m pip install tensorflow tensorflow-probability
$ python -c "import tensorflow_probability"
Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "/home/feickert/.venvs/debug-this/lib/python3.7/site-packages/tensorflow_probability/__init__.py", line 76, in <module>
    from tensorflow_probability.python import *  # pylint: disable=wildcard-import
  File "/home/feickert/.venvs/debug-this/lib/python3.7/site-packages/tensorflow_probability/python/__init__.py", line 23, in <module>
    from tensorflow_probability.python import distributions
  File "/home/feickert/.venvs/debug-this/lib/python3.7/site-packages/tensorflow_probability/python/distributions/__init__.py", line 88, in <module>
    from tensorflow_probability.python.distributions.pixel_cnn import PixelCNN
  File "/home/feickert/.venvs/debug-this/lib/python3.7/site-packages/tensorflow_probability/python/distributions/pixel_cnn.py", line 37, in <module>
    from tensorflow_probability.python.layers import weight_norm
  File "/home/feickert/.venvs/debug-this/lib/python3.7/site-packages/tensorflow_probability/python/layers/__init__.py", line 31, in <module>
    from tensorflow_probability.python.layers.distribution_layer import CategoricalMixtureOfOneHotCategorical
  File "/home/feickert/.venvs/debug-this/lib/python3.7/site-packages/tensorflow_probability/python/layers/distribution_layer.py", line 28, in <module>
    from cloudpickle.cloudpickle import CloudPickler
ImportError: cannot import name 'CloudPickler' from 'cloudpickle.cloudpickle' (/home/feickert/.venvs/debug-this/lib/python3.7/site-packages/cloudpickle/cloudpickle.py)
$ pip list | grep cloudpickle
cloudpickle            1.5.0

Checklist

• Run git fetch to get the most up to date version of master
• Searched through existing Issues to confirm this is not a duplicate issue
• Filled out the Description, Expected Behavior, Actual Behavior, and Steps to Reproduce sections above or have edited/removed them in a way that fully describes the issue