anchor.diff:  
     in lib.mapAttrs testfun envs;
   
  +  # Integration tests involving the package set.
     # All PyPy package builds are broken at the moment
     integrationTests = lib.optionalAttrs (python.pythonAtLeast "3.7"  && (!python.isPyPy)) rec {
       # Before the addition of NIX_PYTHONPREFIX mypy was broken with typed packages
  
positive.code: }:
  
  let
    environmentTests = let
      envs = let
        inherit python;
  
negative.code: # Tests for the Python interpreters, package sets and environments.
  #
  # Each Python interpreter has a `passthru.tests` which is the attribute set
  # returned by this function. For example, for Python 3 the tests are run with
  #
  # $ nix-build -A python3.tests
  #
  { stdenv
  , python
  , runCommand
  