Luckily tox-conda offers another solution:


> conda_deps, which is used to configure which dependencies are installed from conda instead of from pip. All dependencies in conda_deps are installed before all dependencies in deps. If not given, no dependencies will be installed using conda.



github.com/tox-dev/tox-conda/b…

conda_deps running before deps is critical, it's what allows us to pre-require packages.

So my pip/setuptools problem is solved by:

[testenv]
conda_deps =
    pip >= 18
    setuptools >= 42


Now my deps requiring setuptools_scm and my packages packaged with flit can install just fine.

What I will never figure out is why one of the branches did not have this problem.