Support matplotlib 3.8

[bmcfee]

Matplotlib 3.8 removes some (private) functionality that we've been relying on in the display module:

https://github.com/craffel/mir_eval/blob/9be1c5bf9b02b842b22f5b248189fbf88a7e30ac/mir_eval/display.py#L150

https://github.com/craffel/mir_eval/blob/9be1c5bf9b02b842b22f5b248189fbf88a7e30ac/mir_eval/display.py#L273

https://github.com/craffel/mir_eval/blob/9be1c5bf9b02b842b22f5b248189fbf88a7e30ac/mir_eval/display.py#L464

https://github.com/craffel/mir_eval/blob/9be1c5bf9b02b842b22f5b248189fbf88a7e30ac/mir_eval/display.py#L555

https://github.com/craffel/mir_eval/blob/9be1c5bf9b02b842b22f5b248189fbf88a7e30ac/mir_eval/display.py#L623

https://github.com/craffel/mir_eval/blob/9be1c5bf9b02b842b22f5b248189fbf88a7e30ac/mir_eval/display.py#L782-L786

We probably should never have been doing this — my bad! :sweat_smile: — but now we have to remove / rewrite this functionality to maintain compatibility.

This is going to take a bit of work, and I probably can't commit to doing this in the short term. But I'd be happy to help out if anyone wants to take this on.