[PyROOT] Memory leak when iterating over std::map from Python

Reproducer taken from the forum (https://root-forum.cern.ch/t/memory-leak-using-root-std-map-int-t-float-t/47737):

```python
import ROOT
allChannels = ROOT.std.map(’<Int_t,Float_t>’)()
allChannels[3]=5.
N=0
for n in range(10000000):
    for c in allChannels:
        N+=1
```