test: test the code in the README

[tonyandrewmeyer]

Adds a tox environment test-readme (not run by default) that tests (most of) the code blocks in README.md.

Also some corresponding changes to the README:

• Remove all the import ops and import scenario lines - these seem like noise and are implied I think.
• Remove "from charm import Charm" type lines and "class MyCharm(...): ..." lines - these also seem like noise, although they are a little bit less implied. I could add a "here is the code that's used throughout the doc" section at the top if that would be better.
• Flesh out a couple of examples a small amount so that the various asserts work.
• Fix a few bugs that had crept in (the kind of ones that running this tox command would catch).

[PietroPasotti]

TIL: pytest-markdown. Fun stuff!

[benhoyt]

Nice idea, thanks!