Bump isort from 4.3.4 to 4.3.21 in /cvat/requirements (#1692)

Bumps [isort](https://github.com/timothycrosley/isort) from 4.3.4 to 4.3.21.
- [Release notes](https://github.com/timothycrosley/isort/releases)
- [Changelog](https://github.com/timothycrosley/isort/blob/develop/CHANGELOG.md)
- [Commits](https://github.com/timothycrosley/isort/compare/4.3.4...4.3.21)

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

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

cvat/requirements/development.txt

@@ -1,6 +1,6 @@
 -r base.txt
 astroid==2.2.5
-isort==4.3.4
+isort==4.3.21
 lazy-object-proxy==1.3.1
 mccabe==0.6.1
 pylint==2.3.1