Bump imgaug from 0.2.9 to 0.4.0 in /cvat/requirements (#1745)

Bumps [imgaug](https://github.com/aleju/imgaug) from 0.2.9 to 0.4.0.
- [Release notes](https://github.com/aleju/imgaug/releases)
- [Commits](https://github.com/aleju/imgaug/compare/0.2.9...0.4.0)

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

cvat/requirements/base.txt

@@ -42,7 +42,7 @@ tensorflow==1.15.2
 keras==2.3.1
 opencv-python==4.1.0.25
 h5py==2.9.0
-imgaug==0.2.9
+imgaug==0.4.0
 django-cors-headers==3.3.0
 furl==2.0.0
 av==6.2.0