SF patch 670012: Compatibility changes for _strptime.py.

Patch from Brett Cannon:

    First, the 'y' directive now handles [00, 68] as a suffix for the
    21st century while [69, 99] is treated as the suffix for the 20th
    century (this is for Open Group compatibility).

    strptime now returns default values that make it a valid date ...

    the ability to pass in a regex object to use instead of a format
    string (and the inverse ability to have strptime return a regex object)
    has been removed. This is in preparation for a future patch that will
    add some caching internally to get a speed boost.