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datetime.(from_excel, to_excel): drop excel bug workaround

The specification is clear on how to (de)serialize dates in the 1900
date system. Instead of attempting to follow what Excel does (which we
can't do fully anyway), let's follow the specification. This increases
interoperability and allows correct timedelta calculations for time
differences above 60 days.
7 jobs for topic/default/timedelta in 5 minutes and 38 seconds (queued for 1 second)
Status Name Job ID Coverage
  Test
passed test-coverage #155951

00:01:34

95.0%
passed test-pypy3 #155950

00:02:40

passed test-python36 #155946

00:01:20

passed test-python37 #156607

00:02:57

passed test-python38 #155948

00:01:30

passed test-python39 #155949

00:01:09

failed test-python37 #155947

00:02:04