Increase the delay between PubChem retries (#1284)

The live API test still fails in CI. It ends up failing 4 times in a
row:

```
test_geometry_from_pubchem_live_api failed on attempt 1! Retrying!
...
test_geometry_from_pubchem_live_api failed on attempt 2! Retrying!
...
test_geometry_from_pubchem_live_api failed on attempt 3! Retrying!
...
test_geometry_from_pubchem_live_api failed after 4 attempts!
```

Let's increase the retry delay.

Author: mhucka

src/openfermion/chem/pubchem_test.py

@@ -149,7 +149,7 @@ def test_water_2d(self, monkeypatch):
         with pytest.raises(ValueError, match='Incorrect value for the argument structure'):
             _ = geometry_from_pubchem('water', structure='foo')
 
-    @pytest.mark.flaky(retries=3, delay=2, only_on=[pubchempy.ServerBusyError])
+    @pytest.mark.flaky(retries=3, delay=10, backoff=2, only_on=[pubchempy.ServerBusyError])
     def test_geometry_from_pubchem_live_api(self):
         water_geometry = geometry_from_pubchem('water')
         assert len(water_geometry) == 3