Update Mypy version at first opportunity

[parafoxia]

The version of Mypy needs to be updated in the dev requirements file, as some dataclass features are not supported in 0.910 (see python/mypy#10865 and python/mypy#11482). These issues have been fixed in the development version (python/mypy#10867 and python/mypy#11483), but have not been released to PyPI.

8b2a3d9 allows for the use of the development version, but as this slows down the pipeline pretty heavily, the version needs to updated to the latest stable as soon as it's available.