Remove requirement for status attr/name/label to be valid identifier

[samuelallan72]

this:

        if not attribute.isidentifier():
            raise ValueError(
                f"cannot set {attribute!r}={status} on {self}: "
                f"attribute needs to be a valid Python identifier."
            )

There are many cases where a user won't care about using the pool.status_label ... syntax sugar, and instead only want to use pool.set_status(...), pool.get_status(...), etc. In these cases, the validation is confusing and unnecessary. It also becomes annoying and error prone to a user, since now they need to write some function to map names to valid names, and remember to use it everywhere they want to access the status.