To summarize, here's a repro for Dom's case:
```sh
brew install python python3
# Make sure the first python in PATH isn't python3, or the following step will fail.
brew reinstall libmagic --with-python
brew reinstall -s diffoscope
```
=>
```
...
Your PYTHONPATH points to a site-packages dir for Python 2.x but you are running Python 3.x!
...
```
@DomT4 I suppose you installed `libmagic --with-python`? I can reproduce the `python_binary: python` instances if that's the case, but not if `libmagic` is installed without python.
I didn't check the code, but it looks like when dependencies and requirements are collected recursively, the `PythonRequirement` of `libmagic --with-python` is being picked up, and its `env` is apparently overriding that of `Python3Requirement` from `diffoscope`.