Unable to type hint new StringDType

[TomNicholas]

In #107 we now use the new numpy variable-length StringDType to store paths in-memory in the Manifest, but I had to ignore some typing errors to satisfy mypy. Turns out this is an upstream issue (numpy/numpy#26747). I actually don't really understand what they're talking about in that issue, but once there is a clear recommendation we can remove the type: ignores here.

[TomNicholas]

I think this is fixed - #252 seems to work at least.