nix: add shell.nix

shell.nix

@@ -0,0 +1,9 @@
+with import <nixpkgs> {};
+
+let
+  myPython = python36.withPackages(p: [p.click p.graphviz p.trezor p.ed25519 p.pillow]) ;
+in
+  stdenv.mkDerivation {
+    name = "trezor-common-dev";
+    buildInputs = [ myPython python27Packages.demjson graphviz ];
+  }