I've brought up a python 3.8 RockyLinux ST2 using this change, and added some debugging to check that it didn't fall through to the catchall of using standard datetime.
I also did a quick check that I could install ciso8601 on Rocky 8 under python3.9 - and then perform a ciso8601.parse_datetime to check it also worked on python3.9.

I haven't done any benchmarking but the benchmarking of ciso8601 compared to udatetime compared to native python is available on other sites. It generally is much better than native python but not quite as quick as udatetime.

Just to be clear - you mean ISO8601, correct?

ISO8601 is the standard ciso8601 is the python library