TEST: Cap mypy at latest version for longer term fix to #5326

Latest version of dask [uses](https://github.com/dask/distributed/blob/8c81d03c959d9535340576a92bf17f4d28520360/distributed/scheduler.py#L120) `annotation-unchecked` which seems to be a new annotation added [here](https://github.com/python/mypy/pull/13851) and presumably released recently.