REL: v3.3.3

This is the third bugfix release of the 3.3.x series.

This release contains several critical bug-fixes:

* Fix calls to `Axis.grid` with argument `visible=True`.
* Fix fully masked `imshow`.
* Fix inconsistent color mapping in scatter for 3D plots.
* Fix notebook/nbAgg figures when used with ipywidgets in the same cell.
* Fix notebook/nbAgg/WebAgg on older (e.g., Firefox ESR) browsers.
* Fix pcolormesh with `datetime` coordinates.
* Fix performance regression with `datetime`s.
* Fix singular ticks with small log ranges.
* Fix timers/animations on wx and notebook backends.
* Remove certifi as a hard runtime dependency.