Installing pyqt5 with homebrew just for python2
Ugh, of course after trying to fix this for ages, I find the problem 5 min after posting this question!

For anyone else who runs into this:

The problem was that I didn't have sip, which is a requirement, and sip automatically installs for both versions of python too, and so it was sip trying to install python3. sip has its own version of --without-python, but that didn't get passed to it from the pyqt5 install.

tl;dr - install sip separately first:

brew install sip --without-python
brew install pyqt5 --without-python