I don’t know what caused it in your environment but I had a similar error message using python virtual environment. I was not surprised since I tried to use an old python version on my Mac. It was the first time I got this error message and it was just today. I show how I could reproduce it. It might help you or someone else find a solution for you.
✘ ta@ta-mb ~
» virtualenv --python python2 venv
created virtual environment CPython2.7.18.final.0-64 in 129ms
creator CPython2macOsFramework(dest=/Users/ta/venv, clear=False, no_vcs_ignore=False, global=False)
seeder FromAppData(download=False, pip=bundle, setuptools=bundle, wheel=bundle, via=copy, app_data_dir=/Users/ta/Library/Application Support/virtualenv)
added seed packages: pip==20.3.4, setuptools==44.1.1, wheel==0.36.2
» source venv/bin/activate
(venv) ta@ta-mb ~
» pip --version
 21865 killed pip --version
What version of Python are you using?