python api: fix convert_datetime usec case
python api: fix convert_datetime usec case

Pasto which slipped through since no in-tree users.
Renamed a variable to be a bit more descriptive too.

adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp