Fix PyPy3 support and add bug references

Fix #1814

CHANGES

@@ -3,6 +3,13 @@ Flask Changelog
 
 Here you can see the full list of changes between each Flask release.
 
+Version 0.11.2
+--------------
+
+Bugfix release, unreleased
+
+- Fix crash when running under PyPy3, see pull request ``#1814``.
+
 Version 0.11.1
 --------------
 

flask/_compat.py

@@ -65,17 +65,25 @@ def __new__(cls, name, this_bases, d):
 
 
 # Certain versions of pypy have a bug where clearing the exception stack
-# breaks the __exit__ function in a very peculiar way.  This is currently
-# true for pypy 2.2.1 for instance.  The second level of exception blocks
-# is necessary because pypy seems to forget to check if an exception
-# happened until the next bytecode instruction?
+# breaks the __exit__ function in a very peculiar way.  The second level of
+# exception blocks is necessary because pypy seems to forget to check if an
+# exception happened until the next bytecode instruction?
+#
+# Relevant PyPy bugfix commit:
+# https://bitbucket.org/pypy/pypy/commits/77ecf91c635a287e88e60d8ddb0f4e9df4003301
+# According to ronan on #pypy IRC, it is released in PyPy2 2.3 and later
+# versions.
+#
+# Ubuntu 14.04 has PyPy 2.2.1, which does exhibit this bug.
 BROKEN_PYPY_CTXMGR_EXIT = False
 if hasattr(sys, 'pypy_version_info'):
     class _Mgr(object):
         def __enter__(self):
             return self
         def __exit__(self, *args):
-            sys.exc_clear()
+            if hasattr(sys, 'exc_clear'):
+                # Python 3 (PyPy3) doesn't have exc_clear
+                sys.exc_clear()
     try:
         try:
             with _Mgr():