Description of problem:
With the update to python3 only, the deps on the docs package were not updated properly. From the compose report:

[python-doit]                                                                                       
        python3-doit-doc-0.30.3-1.fc28.noarch requires python-doit = 0:0.30.3-1.fc28                

Also, I think you didn't provide proper upgrade paths. Obsoletes (and possibly Provides) should be provided for the "python-doit" name. Otherwise people who installed python-doit will end up with both packages and a conflict over /usr/bin/doit and other files.

Version-Release number of selected component (if applicable):
python-doit-0.30.3-1.fc28