`mypy` CI is failing on `main`

The most recent runs of the `mypy` CI on `main` are failing with:
```
xarray/core/dataset.py:6934: error: Incompatible types in assignment (expression has type "str", variable has type "Optional[Optional[Literal['Y', 'M', 'W', 'D', 'h', 'm', 's', 'ms', 'us', 'ns', 'ps', 'fs', 'as']]]")  [assignment]
```
I'm not sure what changed since the last pass, though.