Unit testing for Python 2.7 with pandapower 2.1.0 fails

[fneum]

Tests for Python 2.7 fail in 7038b76 as it was the first CI run with the recent pandapower v2.1.0 (https://travis-ci.org/PyPSA/PyPSA/jobs/557399790).

While pandapower v2.0.0 already officially dropped Python 2.7 support, the unit tests of pypsa work with up to pandapower v2.0.1 (https://github.com/e2nIEE/pandapower/releases and discussion in e2nIEE/pandapower#224).

1. A proposal would be to fix the version in environment_dev.yaml to pandapower==2.0.1 until the Python 2.7 countdown has run down (https://pythonclock.org/) on January 1, 2020. We make a final release tested for Python 2.7 then, and allow again to pull most recent pandapower.
2. An alternative is to create Python version specific environment_dev.yaml files; e.g. environment_dev_python2.yaml and environment_dev_python3.yaml.
3. Another option is to drop Python 2.7 support right now.

Other opinions/options?

[FabianHofmann]

I guess option 1. is the cleanest one, but I would not wait until the clock ran down but rather drop the support soonly and announcing it in the README.md beforehand.

[coroa]

It's just a test for a deprecated python version.

Just add:

import six
from distutils.version import StrictVersion
pandapower_version = StrictVersion(pp.__version__)

import pytest

@pytest.mark.skipif(six.PY2 and pandapower_version >= '2.0.1',
                    reason="Pandapower 2.0.1 dropped support for Python 2.")
def test_pandapower_case():
   [...]

to test_pf_against_pandapower.py.

[fneum]

Thanks, this is obviously very neat!