Fix requirements and remove deprecated unittest2

[ClementWalter]

No description provided.

[ClementWalter]

Fix for #77

[ClementWalter]

@FabianIsensee here is the minimal change. I have run the test locally and it is all good.

[dyollb]

Why was this never merged or finished?

unittest2 was developed to make it possible to use the standard library unittest (or look-a-like) in Python 2.7 code.
It is outdated and adds obsolete dependencies like these

unittest2==1.1.0
    # via batchgenerators
argparse==1.4.0
    # via unittest2
traceback2==1.4.0
    # via unittest2
linecache2==1.0.0
    # via traceback2

Can I try another attempt to resolve the conflicts and remove this dependency?

I can see there is a rewrite of batchgenerators (batchgeneratorsv2), but nnunet still depends on batchgenerators...