[joss] list supported python versions in pypi

[jdkent]

piggy-backing off of one of @TomDonoghue's comments:

Is there a minimum version of Python3 required? I guess that there is some minimum required sub-version of Py3 required, but it's not clear what that would be, and it might be useful to note that.

you can add supported python versions for pypi, you can add these:

Programming Language :: Python :: 3.5
Programming Language :: Python :: 3.6
Programming Language :: Python :: 3.7

in your setup.py
you can also set python_requires in setup.py to ensure people do not use the incorrect python version with your package.