Django Error on Travis: RuntimeError: populate() isn't reentrant

I am maintaining a project, with Travis CI implemented, and from one day. All my builds start failing without me touching anything or any dependence update ...

I guess that Travis changed something, I tried to investigate, but honestly I have no clue.

If I run a previous build which perfectly worked, now it fails with this everytime I call python manage.py ##whatever##

$ python manage.py collectstatic --noinput
Traceback (most recent call last):
  File "manage.py", line 35, in <module>
    execute_from_command_line(sys.argv)
  File "/home/travis/virtualenv/python3.6.3/lib/python3.6/site-packages/django/core/management/__init__.py", line 381, in execute_from_command_line
    utility.execute()
  File "/home/travis/virtualenv/python3.6.3/lib/python3.6/site-packages/django/core/management/__init__.py", line 357, in execute
    django.setup()
  File "/home/travis/virtualenv/python3.6.3/lib/python3.6/site-packages/django/__init__.py", line 24, in setup
    apps.populate(settings.INSTALLED_APPS)
  File "/home/travis/virtualenv/python3.6.3/lib/python3.6/site-packages/django/apps/registry.py", line 81, in populate
    raise RuntimeError("populate() isn't reentrant")
RuntimeError: populate() isn't reentrant

By the way, I obtain the exact same error with Python 3.6 and 3.5.

$ python manage.py collectstatic --noinput
Traceback (most recent call last):
  File "manage.py", line 35, in <module>
    execute_from_command_line(sys.argv)
  File "/home/travis/virtualenv/python3.5.6/lib/python3.5/site-packages/django/core/management/__init__.py", line 381, in execute_from_command_line
    utility.execute()
  File "/home/travis/virtualenv/python3.5.6/lib/python3.5/site-packages/django/core/management/__init__.py", line 357, in execute
    django.setup()
  File "/home/travis/virtualenv/python3.5.6/lib/python3.5/site-packages/django/__init__.py", line 24, in setup
    apps.populate(settings.INSTALLED_APPS)
  File "/home/travis/virtualenv/python3.5.6/lib/python3.5/site-packages/django/apps/registry.py", line 81, in populate
    raise RuntimeError("populate() isn't reentrant")
RuntimeError: populate() isn't reentrant

And naturally, this issue is impossible to reproduce on my own machine ... It's running just fine locally.

Any idea how to solve this issue ?

from Django Error on Travis: RuntimeError: populate() isn't reentrant