Bump tensorflow from 1.15.2 to 2.12.1

Bumps [tensorflow](https://github.com/tensorflow/tensorflow) from 1.15.2 to 2.12.1.
- [Release notes](https://github.com/tensorflow/tensorflow/releases)
- [Changelog](https://github.com/tensorflow/tensorflow/blob/master/RELEASE.md)
- [Commits](tensorflow/tensorflow@v1.15.2...v2.12.1)

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

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Author: dependabot[bot]

requirements.txt

@@ -39,7 +39,7 @@ scipy==1.4.1
 six==1.15.0
 spektral==0.1.2
 tensorboard==1.15.0
-tensorflow==1.15.2
+tensorflow==2.12.1
 tensorflow-estimator==1.15.1
 termcolor==1.1.0
 tqdm==4.46.0