test: replace tarantool bugs xfail to skip

Tests related to Tarantool datetime bugs [1, 2] are marked with xfail
so it would be convenient to notice when the bug is fixed.
At the same time, tarantool/tarantool CI requires that
tarantool/tarantool-python CI must be green for each PR and it's not
work for a PR with datetime bugs [1, 2] fix: xfail triggers "unexpected
success" and CI fails. To break this codependency, this patch replaces
xfail with skip.

1. tarantool/tarantool#7698
2.tarantool/tarantool#7700

Follows #229

test/suites/test_datetime.py

@@ -388,7 +388,7 @@ def test_python_datetime_subtraction_different_timezones(self):
         self.assertEqual(case['arg_1'] - case['arg_2'], case['res'])
 
     @skip_or_run_datetime_test
-    @unittest.expectedFailure # See https://github.com/tarantool/tarantool/issues/7698
+    @unittest.skip('See https://github.com/tarantool/tarantool/issues/7698')
     def test_tarantool_datetime_subtraction_different_timezones(self):
         case = self.datetime_subtraction_different_timezones_case
 
@@ -524,7 +524,7 @@ def test_python_datetime_addition_winter_time_switch(self):
         self.assertEqual(case['arg_1'] + case['arg_2'], case['res'])
 
     @skip_or_run_datetime_test
-    @unittest.expectedFailure # See https://github.com/tarantool/tarantool/issues/7700
+    @unittest.skip('See https://github.com/tarantool/tarantool/issues/7700')
     def test_tarantool_datetime_addition_winter_time_switch(self):
         case = self.datetime_addition_winter_time_switch_case