Bump tensorflow from 1.15.2 to 2.5.1

Bumps [tensorflow](https://github.com/tensorflow/tensorflow) from 1.15.2 to 2.5.1.
- [Release notes](https://github.com/tensorflow/tensorflow/releases)
- [Changelog](https://github.com/tensorflow/tensorflow/blob/master/RELEASE.md)
- [Commits](tensorflow/tensorflow@v1.15.2...v2.5.1)

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

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Author: dependabot[bot]

requirements.txt

@@ -6,7 +6,7 @@ numpy==1.15.4
 pymongo==3.6.1
 six==1.11.0
 pymagnitude==0.1.120
-tensorflow==1.15.2
+tensorflow==2.5.1
 tensorflow_hub==0.2.0
 sentencepiece==0.1.8
 lime==0.1.1.32