ModuleNotFoundError: No module named 'pip._vendor.six'

I'm new to Ubuntu (20.04.2.0 LTS) and trying to get Python3.9.6 and Pip to work with Virtualenv.

I hope some of you experienced Ubuntu users can help me :)

I have no problem using default Python (3.8.10):

$ python3 -V Python 3.8.10

$ whereis python3 
/usr/bin/python3.8

$ pip3 -V 
pip 21.1.3 from /usr/local/lib/python3.8/dist-packages/pip (python 3.8)

$ virtualenv env_python3.8 
created virtual environment CPython3.8.10.final.0-64 in 155ms

$ source env_python3.8/bin/activate

(env_python3.8) usagi@dev:~$ pip list 
Package Version
pip 20.0.2 pkg-resources 0.0.0
setuptools 44.0.0 wheel 0.34.2

(env_python3.8) usagi@dev:~$ pip install django 
Successfully installed asgiref-3.4.1 django-3.2.5 pytz-2021.1 sqlparse-0.4.1

(env_python3.8) usagi@dev:~$ pip list 
Package Version
asgiref 3.4.1
Django 3.2.5
pip 20.0.2 pkg-resources 0.0.0
pytz 2021.1 setuptools 44.0.0 sqlparse 0.4.1
wheel 0.34.2

These steps work without any problems, but if I do the same steps for Python3.9.6 (from source), it will not work:

$ python3.9 -V 
Python 3.9.6

$ whereis python3 
/usr/local/bin/python3.9

Guess this is one the problems, pip3 looking at Python3.8
$ pip3 -V 
pip 21.1.3 from /usr/local/lib/python3.8/dist-packages/pip (python 3.8)

$ virtualenv -p /usr/local/bin/python3.9 env_python3.9 
created virtual environment CPython3.9.6.final.0-64 in 113ms

$ source env_python3.9/bin/activate

(env_python3.9) usagi@dev:~$ python -V 
Python 3.9.6

(env_python3.9) usagi@dev:~$ pip -V 
Traceback (most recent call last): File "/home/usagi
/env_python3.9/bin/pip", line 5, in from pip._internal.cli.main import main File "/home
/usagi/env_python3.9/lib/python3.9/site-packages/pip/_internal/cli/main.py", line 10, in 
from pip._internal.cli.autocompletion import autocomplete File "/home/usagi/env_python3.9
/lib/python3.9/site-packages/pip/_internal/cli/autocompletion.py", line 9, in from 
pip._internal.cli.main_parser import create_main_parser File "/home/usagi/env_python3.9
/lib/python3.9/site-packages/pip/_internal/cli/main_parser.py", line 7, in from 
pip._internal.cli import cmdoptions File "/home/usagi/env_python3.9/lib/python3.9/site-
packages/pip/_internal/cli/cmdoptions.py", line 24, in from pip._internal.exceptions 
import CommandError File "/home/usagi/env_python3.9/lib/python3.9/site-packages
/pip/_internal/exceptions.py", line 10, in from pip._vendor.six import iteritems 
ModuleNotFoundError: No module named 'pip._vendor.six'

Tried to find information about the error but could not find a solution.

If any of you have had the same problem or recognize the problem and would like to help me, I am grateful for all the help I can get.

Best regards, /Mikael