REL: v3.4.2

This is the second bugfix release of the 3.4.x series.

This release contains several critical bug-fixes:

* Generate wheels usable on older PyPy7.3.{0,1}
* Fix compatibility with Python 3.10
* Add `subplot_mosaic` Axes in the order the user gave them to us
* Correctly handle 'none' *facecolors* in `do_3d_projection`
* Ensure that Matplotlib is importable even if there's no HOME
* Fix `CenteredNorm` with *halfrange*
* Fix bar_label for bars with nan values
* Fix clip paths when zoomed such that they are outside the figure
* Fix creation of `RangeSlider` with *valinit*
* Fix handling of "d" glyph in backend_ps, fixing EPS output
* Fix handling of datetime coordinates in `pcolormesh` with Pandas
* Fix processing of some `errorbar` arguments
* Fix removal of shared polar Axes
* Fix resetting grid visibility
* Fix subfigure indexing error and tight bbox
* Fix textbox cursor color
* Fix TkAgg event loop error on window close
* Ignore errors for sip with no setapi (Qt4Agg import errors)