REL: v3.3.1

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

This release contains several critical bug-fixes:

* fix docstring import issues when running Python with optimization
* fix `hist` with categorical data, such as with Pandas
* fix install on BSD systems
* fix nbagg compatibility with Chrome 84+
* fix ordering of scatter marker size in 3D plots
* fix performance regression when plotting `Path`s
* fix reading from URL in `imread`
* fix several regressions with new date epoch handling
* fix some bad constrained and tight layout interactions with colorbars
* fix use of customized toolbars in TkAgg and WXAgg backends