2-digit years are now converted using rules that are (according to
Fredrik Lundh) recommended by POSIX or X/Open: 0-68 mean 2000-2068,
69-99 mean 1969-1999.

2-digit years are now only accepted if time.accept2dyear is set to a
nonzero integer; if it is zero or not an integer or absent, only year
values >= 1900 are accepted.  Year values 100-1899 and negative year
values are never accepted.

The initial value of time.accept2dyear depends on the environment
variable PYTHONY2K: if PYTHONY2K is set and non-empty,
time.accept2dyear is initialized to 0; if PYTHONY2K is empty or not
set, time.accept2dyear is initialized to 0.