skip m2crypto tests on Python 3

.travis.yml

@@ -16,7 +16,7 @@ before_install:
 script:
   - pep8 .
   - pyflakes .
-  - python tests/nose_plugin.py
+  - python tests/nose_plugin.py -v
   - python tests/test.py -c tests/aes.json
   - python tests/test.py -c tests/aes-ctr.json
   - python tests/test.py -c tests/aes-cfb1.json

shadowsocks/crypto/m2.py

@@ -80,18 +80,30 @@ def run_method(method):
 
 
 def test_aes_128_cfb():
+    if bytes != str:
+        from nose.plugins.skip import SkipTest
+        raise SkipTest
     run_method(b'aes-128-cfb')
 
 
 def test_aes_256_cfb():
+    if bytes != str:
+        from nose.plugins.skip import SkipTest
+        raise SkipTest
     run_method(b'aes-256-cfb')
 
 
 def test_bf_cfb():
+    if bytes != str:
+        from nose.plugins.skip import SkipTest
+        raise SkipTest
     run_method(b'bf-cfb')
 
 
 def test_rc4():
+    if bytes != str:
+        from nose.plugins.skip import SkipTest
+        raise SkipTest
     run_method(b'rc4')