[PATCH] import-checker: ensure cffi is always a system module

[Augie Fackler]

# HG changeset patch
# User Augie Fackler <raf at durin42.com>
# Date 1466729137 14400
#      Thu Jun 23 20:45:37 2016 -0400
# Node ID 45f17e51dba8b6de76c98ade585d8e31a7379d16
# Parent  6d96658a22b0fc1eb210c99c5629dd69fedf3006
import-checker: ensure cffi is always a system module

I've had reports that this is not always happening, so whitelist it
the way we whitelist other problem cases.

diff --git a/contrib/import-checker.py b/contrib/import-checker.py
--- a/contrib/import-checker.py
+++ b/contrib/import-checker.py
@@ -186,6 +186,9 @@ def list_stdlib_modules():
 
     >>> 'cStringIO' in mods
     True
+
+    >>> 'cffi' in mods
+    True
     """
     for m in sys.builtin_module_names:
         yield m
@@ -198,6 +201,8 @@ def list_stdlib_modules():
         yield m
     for m in 'cPickle', 'datetime': # in Python (not C) on PyPy
         yield m
+    for m in ['cffi']:
+        yield m
     stdlib_prefixes = set([sys.prefix, sys.exec_prefix])
     # We need to supplement the list of prefixes for the search to work
     # when run from within a virtualenv.