`migrate` script fails because Python 3.7 doesn't support type hinting

[powergee]

Hello,

First of all, thank you for starting this project. I'm using these docker configuration for my project, and it is much more convenient to deply online judge frontend.

I found an error while generating the schema for the DB by /dmoj/scripts/migrate.

Current Problem

• migrate fails with ImportError
  While executing migrate, it fails with ImportError: cannot import name 'Literal' from 'typing' (/usr/lib/python3.7/typing.py). I googled it, and found that Python 3.7, which is installed by apt-get install python3, doesn't support type hinting by default.

  Error messages

    Traceback (most recent call last):
      File "manage.py", line 17, in <module>
        execute_from_command_line(sys.argv)
      File "/usr/local/lib/python3.7/dist-packages/django/core/management/__init__.py", line 381, in execute_from_command_line
        utility.execute()
      File "/usr/local/lib/python3.7/dist-packages/django/core/management/__init__.py", line 357, in execute
        django.setup()
      File "/usr/local/lib/python3.7/dist-packages/django/__init__.py", line 24, in setup
        apps.populate(settings.INSTALLED_APPS)
      File "/usr/local/lib/python3.7/dist-packages/django/apps/registry.py", line 114, in populate
        app_config.import_models()
      File "/usr/local/lib/python3.7/dist-packages/django/apps/config.py", line 211, in import_models
        self.models_module = import_module(models_module_name)
      File "/usr/lib/python3.7/importlib/__init__.py", line 127, in import_module
        return _bootstrap._gcd_import(name[level:], package, level)
      File "<frozen importlib._bootstrap>", line 1006, in _gcd_import
      File "<frozen importlib._bootstrap>", line 983, in _find_and_load
      File "<frozen importlib._bootstrap>", line 967, in _find_and_load_unlocked
      File "<frozen importlib._bootstrap>", line 677, in _load_unlocked
      File "<frozen importlib._bootstrap_external>", line 728, in exec_module
      File "<frozen importlib._bootstrap>", line 219, in _call_with_frames_removed
      File "/site/judge/models/__init__.py", line 4, in <module>
        from judge.models.comment import Comment, CommentLock, CommentVote
      File "/site/judge/models/comment.py", line 16, in <module>
        from judge.models.contest import Contest
      File "/site/judge/models/contest.py", line 14, in <module>
        from judge.models.problem import Problem
      File "/site/judge/models/problem.py", line 18, in <module>
        from judge.models.profile import Organization, Profile
      File "/site/judge/models/profile.py", line 9, in <module>
        import webauthn
      File "/usr/local/lib/python3.7/dist-packages/webauthn/__init__.py", line 1, in <module>
        from .registration.generate_registration_options import generate_registration_options
      File "/usr/local/lib/python3.7/dist-packages/webauthn/registration/generate_registration_options.py", line 3, in <module>
        from webauthn.helpers import generate_challenge
      File "/usr/local/lib/python3.7/dist-packages/webauthn/helpers/__init__.py", line 11, in <module>
        from .options_to_json import options_to_json  # noqa: F401
      File "/usr/local/lib/python3.7/dist-packages/webauthn/helpers/options_to_json.py", line 3, in <module>
        from .structs import (
      File "/usr/local/lib/python3.7/dist-packages/webauthn/helpers/structs.py", line 2, in <module>
        from typing import List, Literal, Optional
    ImportError: cannot import name 'Literal' from 'typing' (/usr/lib/python3.7/typing.py)
  

Suggestion

How about changing the image of base from debian:buster-slim to python:3.8-slim-buster?