Bump tensorflow-cpu from 2.2.0 to 2.2.1 in /tests (#12)

Bumps [tensorflow-cpu](https://github.com/tensorflow/tensorflow) from 2.2.0 to 2.2.1.
- [Release notes](https://github.com/tensorflow/tensorflow/releases)
- [Changelog](https://github.com/tensorflow/tensorflow/blob/master/RELEASE.md)
- [Commits](tensorflow/tensorflow@v2.2.0...v2.2.1)

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tests/requirements.txt

@@ -1,4 +1,4 @@
-tensorflow-cpu==2.2.0
+tensorflow-cpu==2.2.1
 scipy==1.4.1
 scikit-image
 ipython