regular update for matplotlib 3.9

Compatibility fixes and code formatting

Compatibility fixes for matplotlib 3.5 (#32)

Compatibility fixes

Changed
=======

- We now use the new `convert_coordinate` method that has been introduced with
  psyplot v1.4.1 (see psyplot/psyplot#39 and #30)

Fixed
=====
- psy-simple is now compatible with matplotlib 3.5 (see #31)

Compatibility fixes and LGPL license

As with psyplot 1.4.0, psy-simple is now continuously tested and deployed with CircleCI.

Fixed
-----
-   Compatibility fixes for matplotlib\>=3.3

Added
-----
-   psyplot does now have a CITATION.cff file, see https://citation-file-format.github.io

Changed
-------
-   psy-simple is now officially licensed under LGPL-3.0-only, see [#28](#28)
-   We use CicleCI now for a standardized CI/CD pipeline to build and test the code and docs all at one place, see [#27](#27)

Patch for 3D bounds

New background and mask formatoptions and more options for colorbar b…

…ounds

Improved visualization of unstructured grids

New widgets and fldmean plotters

First official and stable release