Hi there,

while packaging for nixpkgs, we noticed that the test-suite does not execute cleanly.

That is, because here:

1. The hook is defined twice (which is wrong, but doesn't hurt)
2. That hook defines clear=True which has the side effect of removing all environment variables, instead of just the ones that this wants to clear.

This has the side effect of removing the HOME variable, which is required to execute the test suite inside the nix build sandbox.

Would you mind changing that? I could provide a pull request, but that's kind of a trivial change, and I wanted to ask first anyway.