Fix sandboxed build

Dependencies were downloading `setuptools-scm` and `pytest-runner` at build
time because `pypi2nix` does not capture `setup_requires`.

requirements_override.nix

@@ -1,10 +1,51 @@
 { pkgs, python }:
+self: super:
+let
+  # Packages use setuptools-scm to try to infer version from source control metadata (say, git tag).
+  # Authors put setuptools-scm in setup_requires.
+  # Add it manually to affected packages.
+  # NOTE: source tarballs don't have scm metadata.
+  # setuptools-scm will just give up and emit 0.0.0.
+  setuptools-scm = python.mkDerivation {
+    name = "setuptools-scm";
+    src = pkgs.fetchurl {
+      url = "https://files.pythonhosted.org/packages/b2/f7/60a645aae001a2e06cf4b8db2fba9d9f36b8fd378f10647e3e218b61b74b/setuptools_scm-3.5.0.tar.gz";
+      sha256 = "11qs1jvfgflx1msv39jgc6bj9d9a300ra35fwypkr44jayh23psv";
+    };
+  };
 
-self: super: {
-  # Break circular dependency: pytest depends on attrs and attrs depends on
-  # pytest to test itself. It certainly hasn't got it as a runtime dep though,
-  # so remove it.
-  "attrs" = python.overrideDerivation super."attrs" (old: {
-     propagatedBuildInputs = [ self."six" ];
-  });
+  addBuildInputs =
+    pkg: buildInputs:
+      python.overrideDerivation pkg (
+        old: {
+          buildInputs = old.buildInputs ++ buildInputs;
+        }
+      );
+in
+{
+  # Break circular dependency: attrs <-> pytest
+  attrs = python.overrideDerivation super.attrs (
+    old: {
+      propagatedBuildInputs = [ self.six ];
+    }
+  );
+
+  # Break circular dependency: mccabe <-> pytest-runner
+  mccabe = python.overrideDerivation super.mccabe (
+    old: {
+      postPatch = ''
+        substituteInPlace setup.py --replace "setup_requires=['pytest-runner']," "setup_requires=[]," || true
+      '';
+    }
+  );
+
+  # pypi2nix does not handle setup_requires.
+  astroid = addBuildInputs super.astroid [ self.pytest-runner ];
+  pluggy = addBuildInputs super.pluggy [ setuptools-scm ];
+  python-dateutil = addBuildInputs super.python-dateutil [ setuptools-scm ];
+  py = addBuildInputs super.py [ setuptools-scm ];
+  pylint = addBuildInputs super.pylint [ self.pytest-runner ];
+  pytest = addBuildInputs super.pytest [ setuptools-scm ];
+  pytest-runner = addBuildInputs super.pytest-runner [ setuptools-scm ];
+  pytest-pylint = addBuildInputs super.pytest-pylint [ self.pytest-runner ];
 }