Fix another test.

Cython/Debugger/Tests/TestLibCython.py

@@ -14,7 +14,7 @@ from distutils import ccompiler
 
 import runtests
 import Cython.Distutils.extension
-import Cython.Distutils.build_ext
+import Cython.Distutils.old_build_ext as build_ext
 from Cython.Debugger import Cygdb as cygdb
 
 root = os.path.dirname(os.path.abspath(__file__))
@@ -24,10 +24,6 @@ cfuncs_file = os.path.join(root, 'cfuncs.c')
 with open(codefile) as f:
     source_to_lineno = dict((line.strip(), i + 1) for i, line in enumerate(f))
 
-# Cython.Distutils.__init__ imports build_ext from build_ext which means we
-# can't access the module anymore. Get it from sys.modules instead.
-build_ext = sys.modules['Cython.Distutils.build_ext']
-
 
 have_gdb = None
 def test_gdb():