Release candidate 1.9

[danielhuppmann]

Description of PR

This PR bumps the copyright year and adds the release highlights.

[codecov[bot]]

Codecov Report

Merging #746 (c5cca76) into main (61b867b) will not change coverage. The diff coverage is n/a.

@@          Coverage Diff          @@
##            main    #746   +/-   ##
=====================================
  Coverage   95.1%   95.1%           
=====================================
  Files         59      59           
  Lines       6017    6017           
=====================================
  Hits        5724    5724           
  Misses       293     293           

[danielhuppmann]

Successfully published on https://test.pypi.org/project/pyam-iamc/1.9.0rc1/