python context manager behavior changing over python versions

with.py

import sys

class Bar(object):
    def __enter__(self):
        pass

    def __exit__(self, exc, exc_type, tb):
        return True


with Bar():
    raise RuntimeError('foo')

# sys.exc_info() returns (None, None, None) under python 2.6,
# and the RuntimeError under python 2.7
# ... and (None, None, None) under python 3.x
print(sys.exc_info())