Bump cython from 0.29.20 to 0.29.21 in /cvat/requirements (#1872)

Bumps [cython](https://github.com/cython/cython) from 0.29.20 to 0.29.21.
- [Release notes](https://github.com/cython/cython/releases)
- [Changelog](https://github.com/cython/cython/blob/master/CHANGES.rst)
- [Commits](https://github.com/cython/cython/compare/0.29.20...0.29.21)

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cvat/requirements/base.txt

@@ -35,7 +35,7 @@ Shapely==1.7.0
 pdf2image==1.13.1
 pascal_voc_writer==0.1.4
 django-rest-auth[with_social]==0.9.5
-cython==0.29.20
+cython==0.29.21
 matplotlib==3.0.3
 scikit-image==0.15.0
 tensorflow==2.2.0