app: Don't check Python version for every request

Checking for the `basestring` name takes a nonzero amount of time, the
result of the check never changes. As such, is not appropriate to do
this for every request.

src/milla/app.py

@@ -40,6 +40,14 @@ __all__ = [
 ]
 
 
+try:
+    # In Python 2, it could be a str or a unicode object
+    _string = basestring
+except NameError:
+    # In Python 3, we are only interested in str objects
+    _string = str
+
+
 class BaseApplication(object):
     '''Base class for Milla applications
 
@@ -88,12 +96,6 @@ class BaseApplication(object):
 
         # The callable might have returned just a string, which is OK,
         # but we need to wrap it in a Response object
-        try:
-            # In Python 2, it could be a str or a unicode object
-            _string = basestring
-        except NameError:
-            # In Python 3, we are only interested in str objects
-            _string = str
         if isinstance(response, _string) or not response:
             response = request.ResponseClass(response)