Minimize impact of namespace package support for CPython 3.3.

This solves a regression with an admittedly obscure use case involving
Mercurial's demandimport implementation, but it also seems like neater code.

--HG--
branch : distribute
extra : rebase_source : 10fb05d0391607140ced288a2c134f4463eddf5a

pkg_resources.py

@@ -1325,12 +1325,8 @@ class DefaultProvider(EggProvider):
 
 register_loader_type(type(None), DefaultProvider)
 
-try:
-    # CPython >=3.3
+if sys.version_info[:2] >= 3.3:
     import _frozen_importlib
-except ImportError:
-    pass
-else:
     register_loader_type(_frozen_importlib.SourceFileLoader, DefaultProvider)