Support pandas 2.0

[jtilly]

Our CI jobs for pandas 2.0 are currently failing (see, e.g., here).

I see (at least) two issues with supporting pandas 2.0:

• REGR: Index.astype(<numpy string dtype>) started failing pandas-dev/pandas#50127 (open PR here): this issue is causing our CI failures
• API/BUG: infer_dtype_from_scalar with non-nano pandas-dev/pandas#52212 changed how pandas is inferring dtypes from scalars, which results in issues when we partition by a datetime (loading the data eagerly will return a datetime64[ns], loading the data as dask data frame will return a datetime64[s]). I don't think we have tests for this by the way. That we could address by explicitly setting the units to nanoseconds here.