Only require toml for projects with pyproject.toml
    
    Pros:
    
     - projects without pyproject.toml will have 1 less dependency
     - toml will be buildable with pyproject-rpm-macros out of the box
     - easier bootstrap sequence (in theory)
    
    Cons:
    
     - projects with pyproject.toml will have 1 more %generate_buildrequires round