do not cause warning in seaborn

I mean it used to be a simple alias, `self.label = self.label1`. So that shouldn't be a problem. I just wanted to make sure we are aware of the implication that once 3.1 is released thousands of seaborn users will get some warning message.

_Originally posted by @ImportanceOfBeingErnest in https://github.com/matplotlib/matplotlib/pull/10088#issuecomment-470245805_