SF patch #691928: Use datetime in _strptime

SF patch #691928: Use datetime in _strptime
Contributed by Brett Cannon.

To prevent code duplication, I patched _strptime to use datetime's date
object to do Julian day, Gregorian, and day of the week calculations.

Patch also includes new regression tests to test results and the
calculation gets triggered.

Very minor comment changes and the contact email are also changed.