fix: assign icc profile in python bindings

[oallw]

Fixes #173

• Assigning an ICC profile to a LayeredFile instance in Python had no effect. The setter was incorrectly assigning to a temporary return value from the getter instead of calling the proper setter method.
• Add Python unit test to reproduce the issue / avoid future regression.

[EmilDohne]

Thanks a lot for taking a look at this! LGTM, I am surprised this has not been an issue (or at least found) before :)

[EmilDohne]

MacOS CI is failing for other reasons, will fix that and merge separately. As long as nothing else fails I'll merge either way

[oallw]

MacOS CI is failing for other reasons, will fix that and merge separately. As long as nothing else fails I'll merge either way

Amazing, thank you!