pythonPackages.trezor: disable build

It seems as recent versions of `trezor` don't work with python 2.x:

```
/build/trezor-0.9.1/dist /build/trezor-0.9.1
Processing ./trezor-0.9.1-py2-none-any.whl
trezor requires Python '>=3.3' but the running Python is 2.7.15
builder for '/nix/store/aqyxki0ckanjk4r1f0an4kj1w4s3kk4f-python2.7-trezor-0.9.1.drv' failed with exit code 1
cannot build derivation '/nix/store/gp4smkzc9r87lzajs17jnq4rh2ayc5q0-python2.7-keepkey-4.0.0.drv': 1 dependencies couldn't be built
```

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

@@ -1,11 +1,13 @@
 { lib, fetchPypi, buildPythonPackage,
-  protobuf, hidapi, ecdsa, mnemonic, requests, pyblake2, click, libusb1, rlp
+  protobuf, hidapi, ecdsa, mnemonic, requests, pyblake2, click, libusb1, rlp, isPy3k
 }:
 
 buildPythonPackage rec {
   pname = "trezor";
   version = "0.9.1";
 
+  disabled = !isPy3k;
+
   src = fetchPypi {
     inherit pname version;
     sha256 = "a481191011bade98f1e9f1201e7c72a83945050657bbc90dc4ac32dc8b8b46a4";