Bump tensorflow from 1.15 to 2.7.2

Bumps [tensorflow](https://github.com/tensorflow/tensorflow) from 1.15 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@v1.15.0...v2.7.2)

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

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Author: dependabot[bot]

requirements.txt

@@ -5,6 +5,6 @@ pillow==6.1.0
 matplotlib==3.1.0
 opencv-contrib-python==4.1.0.25
 scikit-learn==0.21.2
-tensorflow==1.15
+tensorflow==2.7.2
 tensorflow-gpu==1.15
 keras==2.2.4