Remove deprecated unittest2 lib

[ClementWalter]

I am following from this issue on poetry: python-poetry/poetry#3906

I guess that unittest2 is now deprecated and causes troubles for dependency resolution.
Interestingly enough I see that you are still using old fashioned requirements.txt (which is actually not consistent with a dup). If you would like to, I could upgrade your project to using poetry instead.

[FabianIsensee]

Hi Clement,
thank you for pointing this out. We would like to have additional dependencies, especially ones that we are not familiar with.
I agree that we should not be using requirements.txt anymore. Batchgenerators has a long history of 'do not touch a running system' :-). I will update it to use a regular setup.py shortly. Is there a low-tech way of fxing the unittest2 problem?
Best,
Fabian

[ClementWalter]

Actually poetry is not a dependency but a dependency management system. Currently you have deps defined in setup, in requirements, they do not math and are not locking any deps of your project (deps of deps for instance).

I am going to create another PR to show you how it would be