sdk: make test_batch_span_processor_scheduled_delay a bit more robust

It happened that tests failed because the delay was fired some
microseconds earlier:

>       self.assertGreaterEqual((export_time - start_time) * 1e3, 500)
E       AssertionError: 499.9737739562988 not greater than or equal to 500

Use assertAlmostEqual to accept a similar enough value (delta=20) and
avoid too big values.

Skip tests on windows pypy because of random huge spikes:

E AssertionError: 2253.103017807007 != 500 within 20 delta (1749.1030178070068 difference)

Fix #3911

opentelemetry-sdk/tests/trace/export/test_export.py

@@ -461,6 +461,10 @@ def _target():
 
         span_processor.shutdown()
 
+    @mark.skipif(
+        python_implementation() == "PyPy" and system() == "Windows",
+        reason="This test randomly fails with huge delta in Windows with PyPy",
+    )
     def test_batch_span_processor_scheduled_delay(self):
         """Test that spans are exported each schedule_delay_millis"""
         spans_names_list = []
@@ -482,7 +486,7 @@ def test_batch_span_processor_scheduled_delay(self):
         self.assertTrue(export_event.wait(2))
         export_time = time.time()
         self.assertEqual(len(spans_names_list), 1)
-        self.assertGreaterEqual((export_time - start_time) * 1e3, 500)
+        self.assertAlmostEqual((export_time - start_time) * 1e3, 500, delta=20)
 
         span_processor.shutdown()