Improper NeST installation in old pip version (below version 9.0.0)

TLDR: Old versions of pip (below 9.0.0) can install NeST in python 3.5 and below. This causes issues since NeST requires python 3.6+.

NeST works on python 3.6+ [bcoz we use f-string which was introduced in 3.6]. We had added a check in setup.py, such that pip will complain if we try to install on older versions of python.

We used this argument in setup() to do this. But apparently this is a recently added feature, and hence is recognized by pip version 9.0.0 and above.

So what this leads to is someone with python 3.5 and using old version of pip can install NeST successfully and face issues while importing NeST [In fact, we had one such case reported as well].