Cannot install napari-allencell-segmenter through napari menu

I am a technical writer working on the documentation for a [segmentation tutorial](https://chanzuckerberg.github.io/napari-segmentation-workshop/intro.html) that uses napari-allencell-segmenter.

## Description
*A clear description of the bug*
When I try to install napari-allencell-segmenter through the napari menu (**plugins**>**Install/Uninstall**), I cannot get it to install. I can install it using pip install and then use it through the menu. One of the developers told me that the plugin tries to downgrade napari to 0.4.15 and this may be causing the problem. I have been using it with 0.4.15 after pip installing it and there don't seem to be any problems with using it, just with installing it.

### Error message
There is no error message, the plugin just doesn't install through the napari menu.

## Expected Behavior
*What did you expect to happen instead?*
That the plugin would be installed.

## Reproduction
*Please list the exact steps we can take to reproduce this bug.*
1. Open napari on a Windows machine (I am using Windows 10 Enterprise, Version 10.0.19045, Build 19045).
2. Go to plugins>install/uninstall and search on allencell.
3. Select napari-allencell-segmenter.
4. Click the install button on the right end of the line.
5. Check for installation. (It never installs.)
6. Open an Anaconda prompt and use this command: ```python -m pip install napari==0.4.14```
7. Go back to napari and check for installation.

## Environment
- Operating system (ex. MacOS Big Sur): Windows 10 Enterprise, Version 10.0.19045, Build 19045
- Python version (ex. 3.8): 3.9
- napari version (ex. 0.4.1): 0.4.15
- How did you install the napari viewer (as a bundled app or through command line with `pip`)?: command line