Bump tensorflow from 2.4.0 to 2.5.0 in /doc (#586)

Bumps [tensorflow](https://github.com/tensorflow/tensorflow) from 2.4.0 to 2.5.0.
- [Release notes](https://github.com/tensorflow/tensorflow/releases)
- [Changelog](https://github.com/tensorflow/tensorflow/blob/master/RELEASE.md)
- [Commits](tensorflow/tensorflow@v2.4.0...v2.5.0)

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doc/requirements.txt

@@ -13,5 +13,5 @@ sphinx-autodoc-typehints==1.10.3
 sphinx-copybutton
 sphinx-automodapi
 sphinxcontrib-bibtex==0.4.2
-tensorflow==2.4.0
+tensorflow==2.5.0
 toml