Change delimiter as colon (:) delimiter doesn't work well when partitioning on windows

[cholmes]

Working on geoparquet-io I made some tests for hive partitioning by admin boundary and they failed on windows as : in the file system doesn't work.

I think it'd be good at least for this extension, which will be one of the most common ones that people split on, to have something other than colon.

I think a double underscore would work well, but I'm open to other options. Could also be ok for a single underscore.