python: allow nix build from directory (#781)

python/default.nix

@@ -0,0 +1,20 @@
+with import <nixpkgs> {};
+
+let
+  python = let
+    packageOverrides = self: super: {
+      trezor = super.trezor.overridePythonAttrs(old: rec {
+        version = "master";
+        src =  ./.;
+        doCheck = true; # set to false if you want to skip tests
+        checkPhase = ''
+          runHook preCheck
+          pytest --pyargs tests --ignore tests/test_tx_api.py
+          runHook postCheck
+        '';
+      });
+    };
+  in pkgs.python3.override {inherit packageOverrides; self = python;};
+
+in python.withPackages(ps:[ps.trezor])
+