contextmanager doesn't close connection on exit

[ErwinJunge]

When using Connection as a contextmanager, it doesn't close the connection when the context manager exits (https://github.com/chtd/psycopg2cffi/blob/master/psycopg2cffi/_impl/connection.py#L191). It only happens when it gets garbage collected (https://github.com/chtd/psycopg2cffi/blob/master/psycopg2cffi/_impl/connection.py#L184).

This lead to a bunch of dangling connections for me when running a test suite in pypy where I initialized some stuff in a fixture at the start of each test. Apparently pypy doesn't garbage collect quite as aggresively and I managed to run out of available connections in postgres.

In my opinion it would make a lot of sense to end __exit__ with self._close(), since you're not supposed to use the object after the contextmanager exits anyways.

I'd be happy to submit a PR if you agree.