pyenv recently gained the ability to use (and actually prefer) openssl
1.1 on mac OS.
we need to use python 3.5.3, older is not compatible with openssl 1.1.
with python 3.5.2, python was built without ssl support.
also, we need to set the right PKG_CONFIG_PATH, so borg also uses 1.1.
it seems to keep .tox envs even if one removes stuff from requirements file.
if that stuff causes troubles, as here with pytest-benchmark, the troubles stay...
