No module named 'u2flib_server'

After a clean install on a new project the u2flib_server is missing

Here is the ligne I used to install Django-mfa;

$ pip install django-mfa
Collecting django-mfa
  Downloading django_mfa-2.1-py3-none-any.whl (33 kB)
Requirement already satisfied: Django>=2.2.19 in ~/.venv/lib/python3.7/site-packages (from django-mfa) (2.2.20)
Requirement already satisfied: sqlparse>=0.2.2 in ~/.venv/lib/python3.7/site-packages (from Django>=2.2.19->django-mfa) (0.4.1)
Requirement already satisfied: pytz in ~/.venv/lib/python3.7/site-packages (from Django>=2.2.19->django-mfa) (2021.1)
Installing collected packages: django-mfa
Successfully installed django-mfa-2.1

after stating up the installed_apps, middleware and urls when I run my Django server I get this error stack

  File "~/.venv/lib/python3.7/site-packages/django_mfa/urls.py", line 1, in <module>
    from .views import *
  File "~/.venv/lib/python3.7/site-packages/django_mfa/views.py", line 19, in <module>
    from u2flib_server import u2f
ModuleNotFoundError: No module named 'u2flib_server'

in fact, this line

django-mfa/django_mfa/views.py

Line 19 in f51a06b

from u2flib_server import u2f

need the package python-u2flib-server which is not in the requirements (only in the sandbox/requirements.txt how are not installed via pip install)