CVAT doen't support python 3.6

[MashaSS]

My actions before raising this issue

• Read/searched the docs
• Searched past issues

CONTRIBUTING.md provide information about installation on Ubuntu 18.04 and this platform has a default python 3.6. However merged PR #3164 uses syntax of python 3.8 which is not supported in version 3.6.
Error message:
File "/home/maria/Desktop/3d/cvat/cvat/apps/dataset_manager/annotation.py", line 13, in <module> from cvat.apps.engine.serializers import LabeledDataSerializer File "/home/maria/Desktop/3d/cvat/cvat/apps/engine/serializers.py", line 428 if new_label_for_name := list(filter(lambda x: x.get('id', None) == old_label.id, labels)): ^ SyntaxError: invalid syntax

Your Environment

• Git hash commit (git log -1):
• Docker version docker version (e.g. Docker 17.0.05):
• Are you using Docker Swarm or Kubernetes?
• Operating System and version (e.g. Linux, Windows, MacOS):
• Code example or link to GitHub repo or gist to reproduce problem:
• Other diagnostic information / logs:
  Logs from `cvat` container

Next steps

You may join our Gitter channel for community support.