Robustify the Pubchem live api pytest test (#1276)

The Pubchem server often returns a "server busy" error. The code in
pubchem_test.py would retry the test in those situations (as desired),
but it turned out that the error still got propagated up and cause
pytest to report a failure.

This PR tries to make the test more robust against server busy errors.

---------

Co-authored-by: Pavol Juhas <pavol.juhas@gmail.com>

Author: mhucka

src/openfermion/chem/pubchem_test.py

@@ -12,12 +12,12 @@
 
 """Tests for pubchem.py."""
 
-import time
 import unittest
 from unittest.mock import patch
 
 import numpy
 import pytest
+import pubchempy
 
 from openfermion.chem.pubchem import geometry_from_pubchem
 from openfermion.testing.testing_utils import module_importable
@@ -152,7 +152,7 @@ def test_water_2d(self):
         with pytest.raises(ValueError, match='Incorrect value for the argument structure'):
             _ = geometry_from_pubchem('water', structure='foo')
 
-    @pytest.mark.flaky(retries=3, delay=2)
+    @pytest.mark.flaky(retries=3, delay=2, only_on=[pubchempy.ServerBusyError])
     def test_geometry_from_pubchem_live_api(self):
-        water_geometry = geometry_from_pubchem('water')  # pragma: no cover
-        self.assertEqual(len(water_geometry), 3)  # pragma: no cover
+        water_geometry = geometry_from_pubchem('water')
+        self.assertEqual(len(water_geometry), 3)