`make serve` gives warning about python venv

I have a python venv in the root of (most of) my carpentries lessons. This is required to run the make check-lesson-all and so on.
However, when I run `make serve` I get an error (warning?)

```
** ERROR: directory is already being watched! **

Directory: /home/mbexegc2/Documents/training/course-material/git-course/venv/lib64

is already being watched through: /home/mbexegc2/Documents/training/course-material/git-course/venv/lib

MORE INFO: https://github.com/guard/listen/blob/master/README.md
```

I looked at the linked README but didn't really understand what to do.

It's not really a problem in that the site still builds locally, but it's a bit distracting.

Hopefully this is the correct place for this issue.

Start by reproducing the warning with `make serve` in a lesson containing a root-level Python virtual environment, then compare the watched `venv/lib` and `venv/lib64` paths with the Guard Listen README linked in the issue. Done means the local site still builds and serves without the duplicate-watch warning.

Written by the indexing model from the issue text.