Fix more references to flags defined in both `QtCore.Qt` and `QtWidge…

…ts`.

Qt5: use `qimage2ndarray.array2qimage()` for creating a `QImage` as t…

…his supports PythonQt.

PythonQt has no `QtWidgets` module so don’t try to import it...

PythonQt has no `QtWidgets` module so don’t try to import it...

More PythonQt changes for version 3.0.

REL: v3.0.2

This is the second bug-fix release for the v3.0 series.

 - Un-breaks basemap which was broken by partially restoring
   private APIs for cartopy.
 - Fixes bug in warning code when used in an embedded context.
 - Fixes crash when using Tk and closing the first open window before showing it
 - Many documentation improvements.
 - Restore a corner case on ColorBar tick usage.
 - Change the default behavior of `matplotlib.use` to silently allow
   more 'safe' switching after auto-discovery, but before starting an event loop.
 - Improvements to bounding box calculations.
 - Provide the correct length for RcParams instances.

REL: v3.0.1

This is the first bug fix release for the 3.0 series which fixes
several

 - Fix failure to import bug when used with backend-fallback on Python
   3.6.7 and 3.7.1
 - Fixed a number of failure to import bugs around finding fonts
 - Fix Qt4 backend
 - Fix bug on OSX that recursively searched current directory for fonts
 - Fix bouncing-rocket on OSX when doing backend fallback and not
   selecting OSX
 - Temporarily restore several private APIs to unbreak cartopy
 - Make pyplot more tolerant of varying signatures in 3rd-party
   sub-classe
 - Improve datetime64 unit handling
 - Fixed several poor interactions with tight_layout

REL: v3.0.0

The first release of the Matplotlib 3.0 series

This is the first version of Matplotlib to only support Python 3.

Highlights of this release include:

 - GUI backend is selected at run-time based on what toolkits are
   installed.  A GUI toolkit will not be selected on a headless
   server.
 - New cyclic color map *twilight*
 - Improvements to automatic layout of titles, ticks, and GridSpec
 - Many bug fixes!