Fix typo in MacOS part building.md

[CreativeQuantum]

brew install \
        python@3 \ <--- Forgot the antislash here
        llvm \
        eigen \
        opencascade \
        pkg-config \
        gtk4 \
        gtkmm4 \
        glm \
        range-v3 \
        mimalloc \
        pygobject3 \
        librsvg

[CreativeQuantum]

Had to pip3 install pycairo too.

[vespakoen]

@CreativeQuantum those changes seem good to me, can you addpy3cairo to the homebrew deps as well?

It would also be good to include a note that the macOS version is not really working at the moment (selection is broken because of the 1px line width limitation on macOS).

[CreativeQuantum]

Not necessary it seems.

brew deps pygobject3
[...]
py3cairo
[...]

Another way to fix this error:

File "/Users/creativequantum/Documents/Code/GitHub/dune3d/build/../make_icon_texture_atlas.py", line 5, in <module>
    import cairo
ModuleNotFoundError: No module named 'cairo'

pip3 uninstall pycairo # (because I installed it as explained above)
brew remove --ignore-dependencies pygobject3 py3cairo cairo
brew install pygobject3 py3cairo cairo