Bump tensorflow from 1.15.2 to 2.2.0 in /cvat/requirements (#1761)

Bumps [tensorflow](https://github.com/tensorflow/tensorflow) from 1.15.2 to 2.2.0.
- [Release notes](https://github.com/tensorflow/tensorflow/releases)
- [Changelog](https://github.com/tensorflow/tensorflow/blob/master/RELEASE.md)
- [Commits](https://github.com/tensorflow/tensorflow/compare/v1.15.2...v2.2.0)

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cvat/requirements/base.txt

@@ -38,7 +38,7 @@ django-rest-auth[with_social]==0.9.5
 cython==0.29.20
 matplotlib==3.0.3
 scikit-image==0.15.0
-tensorflow==1.15.2
+tensorflow==2.2.0
 keras==2.4.2
 opencv-python==4.1.0.25
 h5py==2.10.0