Bump mypy; fix mypy failure (#9171) · dask/distributed@7fc0080

Process completed with exit code 128.

"pull_request_target.edited" is not a known webhook name (https://developer.github.com/v3/activity/events/types/)

"pull_request_target.synchronize" is not a known webhook name (https://developer.github.com/v3/activity/events/types/)

"pull_request_target.reopened" is not a known webhook name (https://developer.github.com/v3/activity/events/types/)

"pull_request_target.opened" is not a known webhook name (https://developer.github.com/v3/activity/events/types/)

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