filter superfluous ROOT warnings

```python
import rootpy  # <== this now silences the warnings that would otherwise be logged below
from ROOT import TH1D, TCanvas

a = TH1D("a", "a", 10, 0, 1)
c1 = TCanvas()
a.Draw()

c2 = TCanvas()
b = a.Clone()
b.Draw()
b.GetYaxis().SetLimits(0, 0.5)

# ROOT, why do you warn me here?
a + b
a.Divide(b)
a * b
a - b
```

https://sft.its.cern.ch/jira/browse/ROOT-5249