So if I run brew install python-tk, which should let me build basic GUI apps on Mac with Python, it says:

Error: python@3.9: the bottle needs the Apple Command Line Tools to be installed.
  You can install them, if desired, with:
    xcode-select --install

And running xcode-select --installgives me the following dialogue:

Ludicrous installation time
Ludicrous installation time

80 hours! To install some command-line tools!

(In fact it soon dropped to a more sensible 8 minutes, via 13, but still.)