Bump tensorflow from 1.15.4 to 2.7.2

Bumps [tensorflow](https://github.com/tensorflow/tensorflow) from 1.15.4 to 2.7.2.
- [Release notes](https://github.com/tensorflow/tensorflow/releases)
- [Changelog](https://github.com/tensorflow/tensorflow/blob/master/RELEASE.md)
- [Commits](tensorflow/tensorflow@v1.15.4...v2.7.2)

---
updated-dependencies:
- dependency-name: tensorflow
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

old_repo_ensemble_strategy/requirements.txt

@@ -5,7 +5,7 @@ stockstats
 scikit-learn==0.21.0
 gym==0.15.3
 stable-baselines[mpi]
-tensorflow==1.15.4
+tensorflow==2.7.2
 
 joblib==0.15.1