make 'basestring' usages compatible with Py2/Py3

Cython/Compiler/Main.py

@@ -13,6 +13,11 @@ if sys.version_info[:2] < (2, 6) or (3, 0) <= sys.version_info[:2] < (3, 2):
     sys.stderr.write("Sorry, Cython requires Python 2.6+ or 3.2+, found %d.%d\n" % tuple(sys.version_info[:2]))
     sys.exit(1)
 
+try:
+    from __builtin__ import basestring
+except ImportError:
+    basestring = str
+
 from . import Errors
 # Do not import Parsing here, import it when needed, because Parsing imports
 # Nodes, which globally needs debug command line options initialized to set a

Cython/Distutils/build_ext.py

@@ -15,6 +15,11 @@ from distutils import log
 from distutils.command import build_ext as _build_ext
 from distutils import sysconfig
 
+try:
+    from __builtin__ import basestring
+except ImportError:
+    basestring = str
+
 extension_name_re = _build_ext.extension_name_re
 
 show_compilers = _build_ext.show_compilers

Cython/Shadow.py

 # cython.* namespace for pure mode.
+from __future__ import absolute_import
+
 __version__ = "0.23.beta1"
 
+try:
+    from __builtin__ import basestring
+except ImportError:
+    basestring = str
+
 
 # BEGIN shameless copy from Cython/minivect/minitypes.py
 

Cython/Utils.py

@@ -3,6 +3,13 @@
 #            anywhere else in particular
 #
 
+from __future__ import absolute_import
+
+try:
+    from __builtin__ import basestring
+except ImportError:
+    basestring = str
+
 import os
 import sys
 import re