Changes for making PyMunin Python 3 compatible

[penguinpee]

I'm in the process of writing an nftables plugin for Munin and came across PyMunin. Installing it through pip failed due to Pyhton 2 syntax in setup.py and pymunin/__ini__.py.

With this PR the module should install in a Python 3 environment.

I installed it locally from the repository root using:

pip install --user -e .

I did not check any of the plugins for Python 3 compatibility.