Bump tensorflow from 1.15.0 to 2.3.1

Bumps [tensorflow](https://github.com/tensorflow/tensorflow) from 1.15.0 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@v1.15.0...v2.3.1)

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

Author: dependabot[bot]

requirements.txt

@@ -2,7 +2,7 @@ numpy==1.17.3
 Keras==2.3.1
 matplotlib==3.1.1
 pandas==0.25.2
-tensorflow==1.15.0
+tensorflow==2.3.1
 nltk==3.4.5
 jupyter_core==4.6.1
 jupyter==1.0.0