Dropping support for Python 3.5

[felixdivo]

As mentioned in #191, we might consider dropping support for python 3.5. It reached EOL and, for example, numpy is already dropping it (see here).

[Github accidentally flagged it as a feature since I used the template for that.]

[felixdivo]

I'd also add a badge like this to document the currently supported Python version:

Code:

    <a href="https://www.python.org/downloads/releases">
        <img alt="Python 3.6+" src="https://img.shields.io/badge/python-3.6+-blue.svg">
    </a>

[rtavenar]

Maybe we could announce when releasing our next version (pretty soon hopefully) that this will be the last release supporting python 3.5