pythonPackages.mahotas: fix build, move expression

See https://hydra.nixos.org/build/80828287

Moves `mahotas` out of `python-packages.nix` into its own file and fixes
broken test cases by skipping them using nosetest's `@nottest`
annotation.

These tests broke from time to time in a sandbox and are therefore
considered impure.

Addresses #45960

pkgs/development/python-modules/mahotas/default.nix

@@ -0,0 +1,33 @@
+{ buildPythonPackage, fetchFromGitHub, nose, pillow, scipy, numpy, imread, stdenv }:
+
+buildPythonPackage rec {
+  pname = "mahotas";
+  version = "1.4.2";
+
+  src = fetchFromGitHub {
+    owner = "luispedro";
+    repo = "mahotas";
+    rev = "v${version}";
+    sha256 = "1d2hciag5sxw00qj7qz7lbna477ifzmpgl0cv3xqzjkhkn5m4d7r";
+  };
+
+  # remove this as soon as https://github.com/luispedro/mahotas/issues/97 is fixed
+  patches = [ ./disable-impure-tests.patch ];
+
+  propagatedBuildInputs = [ numpy imread pillow scipy ];
+  checkInputs = [ nose ];
+
+  checkPhase= ''
+    python setup.py test
+  '';
+
+  disabled = stdenv.isi686; # Failing tests
+
+  meta = with stdenv.lib; {
+    description = "Computer vision package based on numpy";
+    homepage = http://mahotas.readthedocs.io/;
+    maintainers = with maintainers; [ luispedro ];
+    license = licenses.mit;
+    platforms = platforms.linux;
+  };
+}