changelog

--HG--
extra : rebase_source : ca6bbd2378e2e67004b236da8a67e6b607aa0ae2
implement 'raise ... from None' in Py3.3

--HG--
extra : rebase_source : 9ad9c4ea052a3eae0888f02a07ba4c40d62ddddf
ignore 'raise ... from None' in Py<3.3
add tests for 'raise ... from None'
changelog