Issue #2706: Add support for dividing a timedelta by another timedelta.

Issue #2706:  Add support for dividing a timedelta by another timedelta.

Adds support for the three division operations:

  - timedelta / timedelta -> float
  - timedelta // timedelta -> int
  - timedelta % timedelta -> timedelta

also adds support for divmod(timedelta, timedelta).

Patch by Victor Stinner, adapted for py3k and extended by Alexander
Belopolsky.