BunchOfFiles writer incompatible with matplotlib-3.4

[johnomotani]

The implementation of matplotlib.animation.FileMovieWriter seems to have changed in matplotlib-3.4, and the _frame_sink member no longer exists. This makes the unit tests fail. I guess BunchOfFiles needs to be updated, maybe following the patters in matplotlib.animation https://matplotlib.org/stable/_modules/matplotlib/animation.html?

[t-makaro]

Dev note: Relevant Matplotlib change: https://github.com/matplotlib/matplotlib/pull/19045

[t-makaro]

Looks like a straight swap of _frame_sink with self._proc.stdin, and maybe removing the finish() function. Looks like this will be backwards compatible with older matplotlib too.