Bump PyPI classifier status up from "Pre-Alpha"?

We currently have the "Pre-Alpha" PyPI classifier in the setup for this repository:
```
"Development Status :: 2 - Pre-Alpha"
```

John [previously suggested](https://github.com/dask/dask-image/pull/306#discussion_r1282273663):
> Maybe worth revisiting if this is still accurate? Perhaps that can be a separate discussion though

This issue is for discussion about whether we should bump to Alpha (or Beta, or whatever is appropriate). I don't have strong feelings about it.

The [full list of PyPi status classifiers](https://pypi.org/classifiers/) is:
* ~~1 - Planning~~
* **2 - Pre-Alpha**
* 3 - Alpha
* 4 - Beta
* 5 - Production/Stable
* 6 - Mature
* 7 - Inactive