Seems to break things for some reason, e.g one of the VPC tests:

======================================================================
ERROR: tests.functional.vpc.TestVPC.test_deploy_nat_gtw
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/nix/store/df6jxnkkn3diyrvrzpqx1gs99j0s9m9x-python2.7-nose-1.3.7/lib/python2.7/site-packages/nose/case.py", line 197, in runTest
    self.test(*self.arg)
  File "/home/amine/src/nixops/tests/functional/vpc.py", line 62, in test_deploy_nat_gtw
    self.depl.deploy(plan_only=True)
  File "/home/amine/src/nixops/nixops/deployment.py", line 1063, in deploy
    self.run_with_notify('deploy', lambda: self._deploy(**kwargs))
  File "/home/amine/src/nixops/nixops/deployment.py", line 1052, in run_with_notify
    f()
  File "/home/amine/src/nixops/nixops/deployment.py", line 1063, in <lambda>
    self.run_with_notify('deploy', lambda: self._deploy(**kwargs))
  File "/home/amine/src/nixops/nixops/deployment.py", line 917, in _deploy
    r.index = self._get_free_resource_index()
  File "/home/amine/src/nixops/nixops/util.py", line 258, in set
    else: self._set_attr(name, x)
  File "/home/amine/src/nixops/nixops/resources/__init__.py", line 80, in _set_attr
    self._set_attrs({name: value})
  File "/home/amine/src/nixops/nixops/resources/__init__.py", line 76, in _set_attrs
    (self.id, n, v))
IntegrityError: FOREIGN KEY constraint failed

since it's not urgent, I might get back to it at some point to investigate further.

@dtzWill This is not during the build, this is while running functional tests that do spin up some machines/resources and test things get deployed/destroyed etc ..

Hello!

Thank you for this PR.

In the past several months, some major changes have taken place in
NixOps:

1. Backends have been removed, preferring a plugin-based architecture.
   Here are some of them:

   • https://github.com/NixOS/nixops-aws
   • https://github.com/NixOS/nixops-hetzner
   • https://github.com/nix-community/nixops-vbox
   • https://github.com/nix-community/nixops-libvirtd
   • https://github.com/nix-community/nixops-datadog

2. NixOps Core has been updated to be Python 3 only, and at the
   same time, MyPy type hints have been added and are now strictly
   required during CI.

This is all accumulating in to what I hope will be a NixOps 2.0
release. There is a tracking issue for that:
#1242 . It is possible that
more core changes will be made to NixOps for this release, with a
focus on simplifying NixOps core and making it easier to use and work
on.

My hope is that by adding types and more thorough automated testing,
it will be easier for contributors to make improvements, and for
contributions like this one to merge in the future.

However, because of the major changes, it has become likely that this
PR cannot merge right now as it is. The backlog of now-unmergable PRs
makes it hard to see which ones are being kept up to date.

If you would like to see this merge, please bring it up to date with
master and reopen it. If the or mypy type checking fails, please
correct any issues and then reopen it. I will be looking primarily at
open PRs whose tests are all green.

Thank you again for the work you've done here, I am sorry to be
closing it now.

Graham