Fix absl-py version to 0.8.1

[oatawa1]

What this patch does to fix the issue.

This PR fix absl-py version to 0.8.1 (the latest version before 0.9)
Because absl-py require <0.9, >=0.7

Note that Blueoil code didn't require absl-py but it required by TensorFlow.

Link to any relevant issues or pull requests.

#1167

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[iizukak]

@oatawa1
There is no problem if we use 0.8.1?
I think some library required 0.9 in dependency and this issue happen.

[oatawa1]

@iizukak I thought the 0.8.1 could work so.

I checked it using pip check
First check which dependency need absl-py

# pip check
tensorflow-metadata 0.23.0 requires absl-py, which is not installed.
tensorflow-gpu 1.15.3 requires absl-py, which is not installed.
tensorflow-datasets 1.3.0 requires absl-py, which is not installed.
tensorboard 1.15.0 requires absl-py, which is not installed.

After I installed the absl-py.

absl-py==0.9.0

# pip check
tensorflow-metadata 0.23.0 has requirement absl-py<0.9,>=0.7, but you have absl-py 0.9.0.

absl-py==0.8.1

# pip check
No broken requirements found.

[iizukak]

@oatawa1 Great.

[iizukak]

OA

[iizukak]

/ready

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[oatawa1]

In our case, the tensorflow-datasets==1.3.0 required absl-py but it did not specify the version so it installed the latest version.
Then the tensorflow-meradata is installed with tensorflow which required < 0.9, so it got error.