You need Python 3.6 or later to run GriSPy. You can have multiple Python versions (3.x) installed on the same system without problems.
The easiest way to install is using pip:
$ pip install grispy
This will install the latest stable version on PIPy.
If you want to use the latest development version from github, unpack or clone the repo on your local machine, change the directory to where setup.py is, and install using setuptools:
$ python setup.py install
$ pip install -e .