Klein routers share too much state between class- and instance-level copies

A use-case to set the stage:

I want to generalize my approach to handle_errors and sub-routes. This means I want to do something like:

@attr.s
class RootAPI:
    remoteErrorHandlerConfig = attr.ib()
    router = Klein()
    @router.route("/subroute", branch=True)
    def subroute(self, request):
        domainObject = SubAPI()
        subRouter = domainObject.router
        subRouter.handle_errors(Exception)(
            lambda other, request, failure: self.errorHandler(request, failure)
        )
        return subRouter.resource()

Then I want to test this thing.

The problem is that SubAPI.router, another Klein object, shares mutable state with all other instance router objects, due to this code. Rather than doing k._error_handlers = self._error_handlers[:], we do k._error_handlers = self._error_handlers.

Perhaps the existing state-sharing behavior has to be preserved for compatibility, but:

1. the attributes aren't exposed as settable via a constructor, so I can't create a new Klein myself
2. the construction of the URL map is class-scope, so there's no way to pull in the work the @route decorator does without re-writing it completely myself
3. some of these attributes are (inconsistently!) private, which means even if I had a constructor where I could pass them, I can't retrieve the thing I want to pass.

There should be some way to derive a bound Klein object from an existing Klein object which doesn't share state and can be configured on a per-instance basis, so that my dependency-injected remoteErrorHandlerConfig can be passed along.

Start with src/klein/app.py at the linked lines 158-163, then trace how the class-scope route decorators build the URL map and how instance routers are created. The work is done when a bound Klein object can be derived from an existing one, retain its routes, and configure mutable state independently per instance without breaking compatibility.