Bump tensorflow from 2.5.1 to 2.5.3 in /backend

Bumps [tensorflow](https://github.com/tensorflow/tensorflow) from 2.5.1 to 2.5.3.
- [Release notes](https://github.com/tensorflow/tensorflow/releases)
- [Changelog](https://github.com/tensorflow/tensorflow/blob/master/RELEASE.md)
- [Commits](tensorflow/tensorflow@v2.5.1...v2.5.3)

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

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Author: dependabot[bot]

backend/requirements.txt

@@ -45,7 +45,7 @@ rsa==4.7.2
 scipy==1.6.1
 six==1.15.0
 structlog==21.1.0
-tensorflow==2.5.1
+tensorflow==2.5.3
 termcolor==1.1.0
 tinycss2==1.1.0
 toml==0.10.2