Update pytest requirement to v4.4.0

[bilelomrani1]

When building from source in dev mode, two warnings are raised:

pytest-cov 2.7.1 has requirement pytest>=3.6, but you'll have pytest 3.5.1 which is incompatible.
pytest-xdist 1.29.0 has requirement pytest>=4.4.0, but you'll have pytest 3.5.1 which is incompatible.

I updated pytest requirement to v4.0.0.

[araffin]

Hello,
Thanks for the pointer, I need to update the docker image first and check that everything is fine (+ ignore some warnings as in https://github.com/HumanCompatibleAI/imitation/blob/master/setup.cfg#L23).
We will need to update the changelog too.