Use np.int64 type for day to nanosecond conversion (NEP50)

[bnavigator]

Fixes #921.

Required due to a change in numpy's type promotion with numpy >= 2: https://numpy.org/devdocs/numpy_2_0_migration_guide.html#changes-to-numpy-data-type-promotion NEP 50 -- Promotion rules for Python scalars

The DAYS_TO_MILLIS constant actually contained the number of nanoseconds in a day, so I renamed it. I hope this is not incorrectly used outside of fastparquet. At least it is not part of the API documentation.

[martindurant]

Looks like its time to ditch the dask CI run, since they have deprecated fastparquet as an engine and apparently broken a load of stuff.

[martindurant]

Thanks for fixing this, by the way :).