python-hetzner: Update to bugfix version 0.7.4.

Fixes pinned CA root certificate and probing for system CA bundle.

Closes NixOS/nixops#354.

I'm using fetchFromGitHub now because it's use of fetchzip is more
stable for autogenerated tarballs from GitHub.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>