Use MSGPACK_PUREPYTHON envvar to test fallback module

Author: methane

.travis.yml

@@ -12,4 +12,4 @@ install:
     - cython --cplus msgpack/_packer.pyx
     - cython --cplus msgpack/_unpacker.pyx
 
-script: "tox"
+script: "tox && MSGPACK_PUREPYTHON=x tox"

msgpack/__init__.py

@@ -2,11 +2,15 @@
 from msgpack._version import version
 from msgpack.exceptions import *
 
-try:
-    from msgpack._packer import pack, packb, Packer
-    from msgpack._unpacker import unpack, unpackb, Unpacker
-except ImportError:
+import os
+if os.environ.get('MSGPACK_PUREPYTHON'):
     from msgpack.fallback import pack, packb, Packer, unpack, unpackb, Unpacker
+else:
+    try:
+        from msgpack._packer import pack, packb, Packer
+        from msgpack._unpacker import unpack, unpackb, Unpacker
+    except ImportError:
+        from msgpack.fallback import pack, packb, Packer, unpack, unpackb, Unpacker
 
 # alias for compatibility to simplejson/marshal/pickle.
 load = unpack