Migration fails with NetBox 3.3

[peteeckel]

I just upgraded my test instance to NetBox version 3.3.0.

When I tried to migrate, I ran into an error when loading NetBox DNS:

(netbox) [root@medusa30 netbox]# /opt/netbox/netbox/manage.py migrate 
Traceback (most recent call last):
  File "/opt/netbox/netbox/manage.py", line 10, in <module>
    execute_from_command_line(sys.argv)
  File "/opt/netbox/lib/python3.8/site-packages/django/core/management/__init__.py", line 446, in execute_from_command_line
    utility.execute()
  File "/opt/netbox/lib/python3.8/site-packages/django/core/management/__init__.py", line 440, in execute
    self.fetch_command(subcommand).run_from_argv(self.argv)
  File "/opt/netbox/lib/python3.8/site-packages/django/core/management/base.py", line 414, in run_from_argv
    self.execute(*args, **cmd_options)
  File "/opt/netbox/lib/python3.8/site-packages/django/core/management/base.py", line 460, in execute
    output = self.handle(*args, **options)
  File "/opt/netbox/lib/python3.8/site-packages/django/core/management/base.py", line 98, in wrapped
    res = handle_func(*args, **kwargs)
  File "/opt/netbox/lib/python3.8/site-packages/django/core/management/commands/migrate.py", line 91, in handle
    self.check(databases=[database])
  File "/opt/netbox/lib/python3.8/site-packages/django/core/management/base.py", line 487, in check
    all_issues = checks.run_checks(
  File "/opt/netbox/lib/python3.8/site-packages/django/core/checks/registry.py", line 88, in run_checks
    new_errors = check(app_configs=app_configs, databases=databases)
  File "/opt/netbox/lib/python3.8/site-packages/django/core/checks/urls.py", line 14, in check_url_config
    return check_resolver(resolver)
  File "/opt/netbox/lib/python3.8/site-packages/django/core/checks/urls.py", line 24, in check_resolver
    return check_method()
  File "/opt/netbox/lib/python3.8/site-packages/django/urls/resolvers.py", line 480, in check
    for pattern in self.url_patterns:
  File "/opt/netbox/lib/python3.8/site-packages/django/utils/functional.py", line 49, in __get__
    res = instance.__dict__[self.name] = self.func(instance)
  File "/opt/netbox/lib/python3.8/site-packages/django/urls/resolvers.py", line 696, in url_patterns
    patterns = getattr(self.urlconf_module, "urlpatterns", self.urlconf_module)
  File "/opt/netbox/lib/python3.8/site-packages/django/utils/functional.py", line 49, in __get__
    res = instance.__dict__[self.name] = self.func(instance)
  File "/opt/netbox/lib/python3.8/site-packages/django/urls/resolvers.py", line 689, in urlconf_module
    return import_module(self.urlconf_name)
  File "/opt/netbox/lib64/python3.8/importlib/__init__.py", line 127, in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
  File "<frozen importlib._bootstrap>", line 1014, in _gcd_import
  File "<frozen importlib._bootstrap>", line 991, in _find_and_load
  File "<frozen importlib._bootstrap>", line 975, in _find_and_load_unlocked
  File "<frozen importlib._bootstrap>", line 671, in _load_unlocked
  File "<frozen importlib._bootstrap_external>", line 843, in exec_module
  File "<frozen importlib._bootstrap>", line 219, in _call_with_frames_removed
  File "/opt/netbox/netbox/netbox/urls.py", line 9, in <module>
    from extras.plugins.urls import plugin_admin_patterns, plugin_patterns, plugin_api_patterns
  File "/opt/netbox/netbox/extras/plugins/urls.py", line 35, in <module>
    urlpatterns = import_object(f"{plugin_path}.api.urls.urlpatterns")
  File "/opt/netbox/netbox/extras/plugins/utils.py", line 31, in import_object
    spec.loader.exec_module(module)
  File "/opt/netbox/lib/python3.8/site-packages/netbox_dns/api/urls.py", line 3, in <module>
    from netbox_dns.api.views import (
  File "/opt/netbox/lib/python3.8/site-packages/netbox_dns/api/views.py", line 8, in <module>
    from netbox_dns.api.serializers import (
  File "/opt/netbox/lib/python3.8/site-packages/netbox_dns/api/serializers.py", line 5, in <module>
    from netbox_dns.api.nested_serializers import (
  File "/opt/netbox/lib/python3.8/site-packages/netbox_dns/api/nested_serializers.py", line 3, in <module>
    from netbox.api import WritableNestedSerializer
ImportError: cannot import name 'WritableNestedSerializer' from 'netbox.api' (/opt/netbox/netbox/netbox/api/__init__.py)

This prevents the migration code from working and NetBox can no longer be restarted successfully when NetBox DNS is enabled.