nix: modify shell.nix to work with pipenv

shell.nix

@@ -1,44 +1,29 @@
 with import <nixpkgs> {};
 
-let
+stdenv.mkDerivation {
-  MyPython = python3.withPackages(ps: [
+  name = "trezor-firmware-dev";
-    ps.Mako
+  buildInputs = [
-    ps.Pyro4
+    SDL2
-    ps.black
+    SDL2_image
-    ps.coverage
+    autoflake
-    ps.ed25519
+    check
-    ps.flake8
+    clang-tools
-    ps.flaky
+    gcc
-    ps.graphviz
+    gcc-arm-embedded
-    ps.isort
+    gnumake
-    ps.mock
+    graphviz
-    ps.munch
+    libusb1
-    ps.pillow
+    openssl
-    ps.pytest
+    pipenv
-    ps.pytest-random-order
+    pkgconfig
-    ps.trezor
+    protobuf
-  ]);
+    scons
-in
+    valgrind
-  stdenv.mkDerivation {
+    zlib
-    name = "trezor-firmware-dev";
+  ];
-    buildInputs = [
+  LD_LIBRARY_PATH="${libusb1}/lib";
-      MyPython
+  shellHook = ''
-      SDL2
+    pipenv shell
-      SDL2_image
+    exit
-      autoflake
+  '';
-      check
+}
-      clang-tools
-      gcc
-      gcc-arm-embedded
-      gnumake
-      graphviz
-      openssl
-      pipenv
-      pkgconfig
-      protobuf
-      python2Packages.demjson
-      scons
-      valgrind
-      zlib
-    ];
-  }