Fix: Remove temporary fixes for pytest

[mpeuster]

pytest 4.x is broke because more-itertools>=6.0.0 dropped python2 support.

Error e.g. def _collate(*iterables, key=lambda a: a, reverse=False):

Reason: pytest-dev/pytest#4770

• https://github.com/sonata-nfv/son-emu/blob/master/Dockerfile#L61

• https://github.com/sonata-nfv/son-emu/blob/master/setup.py#L61

• https://github.com/sonata-nfv/son-emu/blob/master/setup.py#L71

• Containernet: Dockerfile, setup.py, Dockerfile.centos