Add support for Proleptic Julian calendar

[mmejai]

When fetching date of birth (P569) of Pythagoras (Item: Q10261) I got a DatavalueError... unsupported calendarmodel for time datavalue. Can you add/adjust the calender model to support for date of birth BCE. Many thanks.

[dahlia]

As of now Pythagoras (Q10261)'s DOB (P569) on Wikidata has the below value:

{
  "type": "time",
  "value": {
    "time": "-0582-00-00T00:00:00Z",
    "timezone": 0,
    "before": 0,
    "after": 0,
    "precision": 7,
    "calendarmodel": "http://www.wikidata.org/entity/Q1985786"
  }
}

And the calendar model it refers to (Q1985786) apparently not just BCE, but Proleptic Julian calendar. It means we need to handle leap years in a different way to how Python's datetime (which assumes the current Gregorian calendar always was according to the docs) deal with them.

What I mean is not I won't implement this calendar model, but just it would take a long time. However, if somebody sends a patch to implement this I'm going to happily review it!