test_DateValue overflows on 32 bit arches

[s-t-e-v-e-n-k]

On 32 bit architectures, the following test fails due to an overflow error.

[  282s] ======================================================================
[  282s] ERROR: test_DateValue (test_value.TestValueModule.test_DateValue) (date=datetime.date(2039, 1, 1))
[  282s] ----------------------------------------------------------------------
[  282s] Traceback (most recent call last):
[  282s]   File "/home/abuild/rpmbuild/BUILD/python-exiv2-0.19.2-build/python-exiv2-0.19.2/tests/test_value.py", line 446, in test_DateValue
[  282s]     self.do_test_DateValue(date)
[  282s]     ~~~~~~~~~~~~~~~~~~~~~~^^^^^^
[  282s]   File "/home/abuild/rpmbuild/BUILD/python-exiv2-0.19.2-build/python-exiv2-0.19.2/tests/test_value.py", line 433, in do_test_DateValue
[  282s]     seconds = time.mktime(time.struct_time(
[  282s]         (py_date.year, py_date.month, py_date.day, 0, 0, 0, 0, 0, 0)))
[  282s] OverflowError: mktime argument out of range

[jim-easterbrook]

At first I thought this might be related to #44, but it's the Python time.mktime that's failing. Hopefully there won't be any 32-bit machines still running by 2039.