Improve discoverability for Python 2 compatibility code.

setup.py

@@ -57,6 +57,7 @@ def read(*rnames):
     install_requires=[
         'zope.interface',
         'setuptools',
+        'six',
     ],
     extras_require=dict(test=['zope.testing']),
 )

src/martian/compat3.py

@@ -1,4 +1,4 @@
-import sys
+import six
 import types
 
 
@@ -8,7 +8,7 @@
     CLASS_TYPES = (type,)
 
 
-if sys.version_info[0] < 3:
+if six.PY2:
     str = unicode  # NOQA
 else:
     str = str

src/martian/directive.py

@@ -1,3 +1,4 @@
+import six
 import sys
 import inspect
 
@@ -262,7 +263,7 @@ def __init__(self, *args, **kw):
     # To get a correct error message, we construct a function that has
     # the same signature as factory(), but without "self".
     def check_factory_signature(self, *arguments, **kw):
-        if sys.version_info.major == 2:
+        if six.PY2:
             (args, varargs, varkw, defaults) = inspect.getargspec(self.factory)
             argspec = inspect.formatargspec(args[1:], varargs, varkw, defaults)
         else:

src/martian/testing.py

@@ -1,3 +1,4 @@
+import six
 import sys
 from types import ModuleType
 from types import FunctionType
@@ -65,7 +66,7 @@ def __init__(cls, classname, bases, dict_):
         return type.__init__(cls, classname, bases, dict_)
 
 
-if sys.version_info[0] < 3:
+if six.PY2:
 
     class FakeModuleObject(object):
         pass

src/martian/util.py

@@ -15,6 +15,7 @@
 """
 
 import re
+import six
 import sys
 import inspect
 
@@ -26,15 +27,15 @@
 
 def not_unicode_or_ascii(value):
     # python3 compatibility
-    if sys.version_info < (3,) and isinstance(value, unicode):  # NOQA
+    if six.PY2 and isinstance(value, unicode):  # NOQA
         return False
     if not isinstance(value, str):
         return True
     return is_not_ascii(value)
 
 
-# extra compatibility for python3.2
-if sys.version_info < (3,):
+# extra compatibility for python2.7
+if six.PY2:
     is_not_ascii = re.compile(eval(r'u"[\u0080-\uffff]"')).search
 else:
     is_not_ascii = re.compile(eval(r'"[\u0080-\uffff]"')).search