Upgrade django from 1.8 to 1.9 - issue

I am upgrading from django 1.8.4 to 1.9.13. I installed django 1.9.3 and my application stops working, it displays the following error:

Unhandled exception in thread started by <function check_errors.<locals>.wrapper at 0x7effd13cbbf8>

Traceback (most recent call last):

  File "/usr/local/lib/python3.4/dist-packages/django/utils/autoreload.py", line 226, in wrapper

    fn(*args, **kwargs)

  File "/usr/local/lib/python3.4/dist-packages/django/core/management/commands/runserver.py", line 109, in inner_run

    autoreload.raise_last_exception()

  File "/usr/local/lib/python3.4/dist-packages/django/utils/autoreload.py", line 249, in raise_last_exception

    six.reraise(*_exception)

  File "/usr/local/lib/python3.4/dist-packages/django/utils/six.py", line 685, in reraise

    raise value.with_traceback(tb)

  File "/usr/local/lib/python3.4/dist-packages/django/utils/autoreload.py", line 226, in wrapper

    fn(*args, **kwargs)

  File "/usr/local/lib/python3.4/dist-packages/django/__init__.py", line 18, in setup

    apps.populate(settings.INSTALLED_APPS)

  File "/usr/local/lib/python3.4/dist-packages/django/apps/registry.py", line 85, in populate

    app_config = AppConfig.create(entry)

  File "/usr/local/lib/python3.4/dist-packages/django/apps/config.py", line 90, in create

    module = import_module(entry)

  File "/usr/lib/python3.4/importlib/__init__.py", line 109, in import_module

    return _bootstrap._gcd_import(name[level:], package, level)

  File "<frozen importlib._bootstrap>", line 2254, in _gcd_import

  File "<frozen importlib._bootstrap>", line 2237, in _find_and_load

  File "<frozen importlib._bootstrap>", line 2226, in _find_and_load_unlocked

  File "<frozen importlib._bootstrap>", line 1200, in _load_unlocked

  File "<frozen importlib._bootstrap>", line 1129, in _exec

  File "<frozen importlib._bootstrap>", line 1471, in exec_module

  File "<frozen importlib._bootstrap>", line 321, in _call_with_frames_removed

  File "/home/me/work/test_upgrade_django_to_1_11/be/myapp/__init__.py", line 1, in <module>

    import myapp.signals

  File "/home/me/work/test_upgrade_django_to_1_11/be/myapp/signals.py", line 6, in <module>

    from django.contrib.auth.models import User

  File "/usr/local/lib/python3.4/dist-packages/django/contrib/auth/models.py", line 4, in <module>

    from django.contrib.auth.base_user import AbstractBaseUser, BaseUserManager

  File "/usr/local/lib/python3.4/dist-packages/django/contrib/auth/base_user.py", line 49, in <module>

    class AbstractBaseUser(models.Model):

  File "/usr/local/lib/python3.4/dist-packages/django/db/models/base.py", line 94, in __new__

    app_config = apps.get_containing_app_config(module)

  File "/usr/local/lib/python3.4/dist-packages/django/apps/registry.py", line 239, in get_containing_app_config

    self.check_apps_ready()

  File "/usr/local/lib/python3.4/dist-packages/django/apps/registry.py", line 124, in check_apps_ready

    raise AppRegistryNotReady("Apps aren't loaded yet.")

django.core.exceptions.AppRegistryNotReady: Apps aren't loaded yet.

In my setting I have:
(...)
INSTALLED_APPS = ( 

    'django.contrib.auth', 

    'django.contrib.contenttypes', 

    'django.contrib.sessions', 

    'django.contrib.sites', 

    'django.contrib.messages', 

    'django.contrib.staticfiles', 

    'django.contrib.admin', 

    'myapp', 

    #'debug_toolbar', 

    'django_extensions', 

    'corsheaders', 

    'watson', 

    'raven.contrib.django.raven_compat', 

    #'django.contrib.admindocs', 

    'django_mailbox', 

    'django.contrib.postgres', )
(...)

I'm using python3.4 and postgresql9.3

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