Support intentionally targeting a venv directory

Usually, people don't want to introduce bugs into a virtual environment. It's not usually under version control, and people don't usually want bugs in library code.

However, one use case is to learn how to deal with bugs in dependencies. So there are some use cases where we want to allow this. Here's an example of a command I'd like to run:

```sh
$ py-bugger --target-dir .venv/lib/python3.12/site-packages/plotly -n 100
Unable to introduce any of the requested bugs.
```

The `-n 100` arg is to hopefully end up with a bug that my small teaching project actually hits. I believe we get the current output because when building `py_files`, we filter out any .py files with venv in the path.

Tasks
---

- [ ] Failing test. Make a copy of one of the current sample scripts, put it in a subdir with venv in the name, it should not get modified. May need an e2e test for this?
- [ ] Implement a flag --modify-venv, which explicitly states that the user wants to allow modifications to venv files.
- [ ] If this flag is set, don't filter out venv dirs.
- [ ] Passing test.
- [ ] Merge
- [ ] Release
- [ ] Announce