policy: add cffi policy for PyPy

This adds cffi policy in the case where we don't want to use C modules,
but instead we're happy to rely on cffi (bundled with pypy)

mercurial/__init__.py

@@ -59,7 +59,7 b' class hgimporter(object):'
                 return zl
 
             try:
-                if modulepolicy == 'py':
+                if modulepolicy in policy.policynoc:
                     raise ImportError()
 
                 zl = ziploader('mercurial')
@@ -86,7 +86,7 b' class hgimporter(object):'
         stem = name.split('.')[-1]
 
         try:
-            if modulepolicy == 'py':
+            if modulepolicy in policy.policynoc:
                 raise ImportError()
 
             modinfo = imp.find_module(stem, mercurial.__path__)

mercurial/policy.py

@@ -14,10 +14,15 b' import sys'
 #
 #    c - require C extensions
 #    allow - allow pure Python implementation when C loading fails
+#    cffi - required cffi versions (implemented within pure module)
+#    cffi-allow - allow pure Python implementation if cffi version is missing
 #    py - only load pure Python modules
 #
 # By default, require the C extensions for performance reasons.
 policy = 'c'
+policynoc = ('cffi', 'cffi-allow', 'py')
+policynocffi = ('c', 'py')
+
 try:
     from . import __modulepolicy__
     policy = __modulepolicy__.modulepolicy
@@ -29,7 +34,7 b' except ImportError:'
 # The canonical way to do this is to test platform.python_implementation().
 # But we don't import platform and don't bloat for it here.
 if '__pypy__' in sys.builtin_module_names:
-    policy = 'py'
+    policy = 'cffi'
 
 # Our C extensions aren't yet compatible with Python 3. So use pure Python
 # on Python 3 for now.