fix: add mypy option in pyproject to avoid module import errors

[simon-mazenoux]

Add a mypy option in pyproject file to avoid module import errors from DIRAC

(see https://mypy.readthedocs.io/en/stable/running_mypy.html#missing-imports)

[chrisburr]

Closing as this was included in https://github.com/DIRACGrid/diracx/pull/55/