Add explicit error for assigning to settings attribute of state machine

[nmbrgts]

This PR adds an explicit error to help prevent users from mistakenly assigning settings to the settings attribute of their state machine class. Ex:

class Machine(GenericStateMachine):
     pass
Machine.settings = settings(...)  # will now raise an error

I'm getting a strange error on when I run ./build.sh check-coverage locally. I'm interested to see if this error shows up in CI.

Closes #1643.

[Zac-HD]

• Needs release notes in RELEASE.rst, for a minor release because we're adding a new error
• We only need an error if the value being assigned is a settings instance.
• I'd name it StateMachineMeta, which fits everything onto one line and fixes the mypy complaints.
• A shorter message would also be fine, for example:

class StateMachineMeta(type):

    def __init__(self, *args, **kwargs):
        super(StateMachineMeta, self).__init__(*args, **kwargs)

    def __setattr__(self, name, value):
        if name == 'settings' and isinstance(value, Settings):
            raise AttributeError(
                'Assigning `{cls}.settings = {value} does nothing.  Assign '
                'to {cls}.TestCase.settings, or use @{value} as a decorator '
                'on the {cls} class.'.format(cls=self.__name__, value=value)
            )
        return type.__setattr__(self, name, value)


class GenericStateMachine(
    StateMachineMeta('GenericStateMachine', (object,), {})  # type: ignore
):

(and here's how you enable syntax highlighting for code blocks)

Otherwise looks good, and having a test is great!

[nmbrgts]

🤦‍♂️ I need two spaces here!

[Zac-HD]

And only four at the start of the line, but Mypy doesn't care about either of those.

[nmbrgts]

I'm not sure how I missed that! Thanks!

[Zac-HD]

Nearly there!

[Zac-HD]

Use class StateMachine(GenericStateMachine): to avoid the class-defines-no-rules error.

[Zac-HD]

Instead of saying that the error directs users to correct assignment methods, we should just do that here too. e.g.

...classes' settings attribute, which is a no-op (:issue:`1643`). Instead, users should assign to ``SomeStateMachine.TestCase.settings``, or use ``@settings(...)`` as a class decorator to handle this automatically.

[nmbrgts]

That reads a lot better than what I wrote. I think I forgot my audience a bit when I was writing these notes haha

[Zac-HD]

LGTM - I'll merge later today once the build is finished 🎉

[nmbrgts]

Thanks Zac! And thanks for all the help along the way!