`tldextract` uses multiple cache directories in `XDG_CACHE_DIR`

After running a bunch of test suites in the past, I ended up having the following directories created in my `~/.cache`:

```
3.10.12.final__.venv__7f37a3__tldextract-3.4.4
3.10.15.final__py310__49acc5__tldextract-5.1.3
3.10.4.final__.venv__7f37a3__tldextract-3.3.0
3.11.4.final__py311__53cd85__tldextract-3.4.4
3.13.9.final__.venv__4f207c__tldextract-5.3.0
3.14.0.final__.venv__4f207c__tldextract-5.3.0
```

Using multiple cache keys in a top-level directory name seems wrong, as it means that users may end up with a lot of stale directories spread all over the place. Could you please move them to a single directory, and ideally add some functionality to clean old entries up?