macOS ships with a stub `/usr/bin/python3` which by default opens a
dialog to install the command line tools. As we run `python3` initially
at launch, this causes the dialog to appear on first run of fish, if the
command line tools are not installed.
Fix this by detecting the case of `/usr/bin/python3` on Darwin without
the command line tools installed, and do not offer that as a viable
python.
This runs build_tools/style.fish, which runs clang-format on C++, fish_indent on fish and (new) black on python.
If anything is wrong with the formatting, we should fix the tools, but automated formatting is worth it.