[release] compatibility fixes and code formatting
this MR merges the changes from the develop branch into master. In particular it implements compatibility fixes and the following changes
- migrate to psyplot-plugin-template !36 (merged)
- fix image comparison and add reference and test images as CI artifacts !37 (merged)
- Update psyplot min version !41 (merged)
- fixed compatiblity with latest matplotlib version !38 (merged)
Edited by Philipp S. Sommer