Bump tensorflow-cpu from 2.2.1 to 2.4.0 in /tests (#15)

Bumps [tensorflow-cpu](https://github.com/tensorflow/tensorflow) from 2.2.1 to 2.4.0.
- [Release notes](https://github.com/tensorflow/tensorflow/releases)
- [Changelog](https://github.com/tensorflow/tensorflow/blob/master/RELEASE.md)
- [Commits](tensorflow/tensorflow@v2.2.1...v2.4.0)

Signed-off-by: dependabot[bot] <support@github.com>

Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

tests/requirements.txt

@@ -1,5 +1,5 @@
 numpy>=1.16.0,<1.19.0
-tensorflow-cpu==2.2.1
+tensorflow-cpu==2.4.0
 scipy==1.4.1
 scikit-image
 ipython