use tox for testing

[jbweston]

tox has a couple of benefits:

• transparent testing against python/dependency version matrices
• a single configuration file (many other tools in our testing pipeline e.g. flake8 read tox.ini)
• can also build documentation, ensuring that the correct environment is used.

Check out https://github.com/jbweston/qsim/ to see how it looks.

[basnijholt]

That sounds like a good idea.

Will you open a PR?

[jbweston]

That sounds like a good idea.

Will you open a PR?

Eventually, yes.

[basnijholt]

I've made a start in #247