dateutils: consolidate localtime_to_utc
- Moves two "equivalent" localtime to/from utc functions from BibFormat and oairepository into dateutils.
- These functions also now accepts dates before 1900.
- Adds new tests for these functions.
- For Python 2.4 compatibility - adds datetime.strptime.
Co-authored-by: Alessio Deiana <alessio.deiana@cern.ch>