[REF] Remove #type: ignore from SLM class

[ReinderVosDeWael]

Currently, mypy crashes on SLM.py due to an import inside a class. This issue is tracked at mypy/issuse/10521.

Once mypy resolves it issue, this issue can be solved by simply removing the following: https://github.com/MICA-MNI/BrainStat/blob/master/brainstat/stats/SLM.py#L1

[zihuaihuai]

mypy is temporary removed now, will mark this issue as add mypy back