Python 2.7 support

[zStupan]

Python 2.7 has reached end of life, and numpy stopped supporting it since numpy 1.17, I think we should do the same in NiaPy 2.0

Thoughts?

[firefly-cpp]

Good point! I totally support this idea.

@GregaVrbancic what about you?

[sisco0]

Hello, please take a look at my modified CI version.
I got rid of Python 2.7 and used Python 3.8 in Windows and Linux.
If that is ok I could create a pull request for it.
You could get an idea of the fork at this comment, where I also link my fork branch.
#297 (comment)
I think that the project should migrate to Poetry as well, as now it seems like the standard for Python projects.
It should be studied if different versions of Python should be used for testing or not, if we use 3.8 (and possibly but optionally 3.9) that should be enough taking into account the people who uses our solution (mostly academic research people).
On this behalf, I remember we had a conversation on porting this library to C++ while creating Python bindings for it, that could lead to a robust solution as well.

[GregaVrbancic]

Sure, we can drop the 2.7 support, this one is easy, but migration to the poetry package manager is a bit harder since all the scripts must be modified.

Regarding the testing versions of Python, in my opinion, we should test against all major versions. It is not the matter of who uses this package the most. On the other hand, we cannot be really sure that the academics really use the latest versions of Python. In my experience, it is quite the opposite, the academics tend to use older versions of Python.

[sisco0]

This issue could be closed with a32d637.