Update for gym==0.17.2 and scikit-image==0.17.2

[tkukurin]

max_episode_steps raises if info is None [1]. skimage import fix.

[1] https://github.com/openai/gym/blob/master/gym/wrappers/time_limit.py#L19

[tkipf]

Thanks for the PR. Does the max_episode_steps issue also affect OpenAI Gym version 0.12.0 (i.e., the version we use in this repository) or only later versions? I was also wondering why you added the from skimage import draw import, as the alias draw is not used in the rest of the file.

[tkukurin]

Sorry, it completely slipped my mind I'm running an environment with latest versions installed (gym==0.17.2 and scikit-image==0.17.2).

gym==0.12 works both when returning None or an empty dict from step().
With skimage there is an import issue and skimage.draw needs to be explicitly imported (I fixed it stylistically in the latest commit).

Basically this PR would update block_pushing to work with latest versions of the two libraries so feel free to disregard.

[tkipf]

Thanks for the explanation! I'll leave this PR open in case someone would want to use the code base with more recent versions of gym and scikit-image

[tkukurin]

Makes sense :)
Updated title and README to reflect actual change.

[tkipf]

Great, thanks!