Add monitoring of dependencies

[behrtam]

To avoid manually checking for changed dependencies, now that we introduced version pinning (#305), maybe we could use https://requires.io to do that for us. As you can see in my fork the versions are extracted successfully.

There are three ways to get notified:

• include a badge into our readme
• email notifications
• automatic pull request

As we only use those packages for testing my feeling is that the badge is sufficient and maybe emails to the track maintainers. It is free for public repos but it can only be activated by administrators.

[kytrinyx]

I'd be happy to activate it. Ill take a look at it today.

[kytrinyx]

OK, I think that should work. Here's the badge markdown:

[![Requirements Status](https://requires.io/github/exercism/xpython/requirements.svg?branch=master)](https://requires.io/github/exercism/xpython/requirements/?branch=master)

[behrtam]

Thank you.