Bump keras from 2.2.5 to 2.3.1 in /cvat/requirements (#1689)

Bumps [keras](https://github.com/keras-team/keras) from 2.2.5 to 2.3.1.
- [Release notes](https://github.com/keras-team/keras/releases)
- [Commits](https://github.com/keras-team/keras/compare/2.2.5...2.3.1)

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Co-authored-by: dependabot-preview[bot] <27856297+dependabot-preview[bot]@users.noreply.github.com>

cvat/requirements/base.txt

@@ -39,7 +39,7 @@ cython==0.29.13
 matplotlib==3.0.3
 scikit-image==0.15.0
 tensorflow==1.15.2
-keras==2.2.5
+keras==2.3.1
 opencv-python==4.1.0.25
 h5py==2.9.0
 imgaug==0.2.9