venv not always a panacea for system python

  $ docker run --rm -it python bash
  [...]

  root@211646f0fa99:/# which python
  /usr/local/bin/python

  root@211646f0fa99:/# python -m venv .venv

  root@211646f0fa99:/# source .venv/bin/activate

  (.venv) root@211646f0fa99:/# which python
  /.venv/bin/python

  (.venv) root@211646f0fa99:/# ls -l "$(which python)"
  lrwxrwxrwx 1 root root 21 May  7 14:47 /.venv/bin/python -> /usr/local/bin/python