What happened: I am a library developer for pymech and I added a (unreleased) xarray backend following the documentation and rioxarray's implementation. This works with xarray==0.19.0, but with the xarray==0.20.0 it no longer works. Could someone clarify what has changed?

What you expected to happen:

Backends can be discovered while using xr.open_dataset(...) with or without using the engine= keyword argument.

Minimal Complete Verifiable Example:

Since pymech's xarray backend is unreleased, I will demonstrate it via rioxarray:

```py

test.py

from xarray.backends import plugins

print("engines =", plugins.list_engines()) plugins.get_backend("rasterio") ```

```sh $ pip install xarray==0.19.0 rioxarray $ python test.py engines = {'rasterio': <rioxarray.xarray_plugin.RasterioBackend object at 0x7f7adca15d00>, 'store': <xarray.backends.store.StoreBackendEntrypoint object at 0x7f7adca647c0>}

$ pip install xarray==0.20.0 $ python test.py engines = {'store': <xarray.backends.store.StoreBackendEntrypoint object at 0x7fd99138efa0>} Traceback (most recent call last): File "<string>", line 1, in <module> File "/tmp/venv/lib/python3.9/site-packages/xarray/backends/plugins.py", line 164, in get_backend raise ValueError( ValueError: unrecognized engine rasterio must be one of: ['store']

```

Anything else we need to know?:

Environment: