Bump tensorflow from 2.0.0 to 2.3.1

Bumps [tensorflow](https://github.com/tensorflow/tensorflow) from 2.0.0 to 2.3.1.
- [Release notes](https://github.com/tensorflow/tensorflow/releases)
- [Changelog](https://github.com/tensorflow/tensorflow/blob/master/RELEASE.md)
- [Commits](tensorflow/tensorflow@v2.0.0...v2.3.1)

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

requirements.txt

@@ -219,7 +219,7 @@ tb-nightly==1.14.0a20190301
 tblib==1.6.0
 tensorboard==2.0.2
 tensorboardX==2.0
-tensorflow==2.0.0
+tensorflow==2.3.1
 tensorflow-estimator==2.0.1
 tensorflow-hub==0.7.0
 termcolor==1.1.0