REL: v3.6.3

This is the third bugfix release of the 3.6.x series.

This release contains several bug-fixes and adjustments:

* Fix Artist removal from `axes_grid1` Axes classes
* Fix `inset_locator` in subfigures
* Fix `scatter` on masked arrays with units
* Fix colorbar ticks with log norm contours
* Fix deprecation warnings in GTK4 backend
* Fix using relative paths in `HTMLWriter`
* Improve failure message from rcParams string validation for tuple
  inputs
* Improve performance of QtAgg backends
* No longer modify `pil_kwargs` argument to `imsave` and `savefig`